Back-up/supplemental heat

does the boiler run non stop on the coldest days? If not it may have some addition capacity to add a hydronic heater.

Can you get under the glass wall section of the home? You could cut in a few kickspace heaters to blow warm air across the glass. They may work at the same radiant temperature, they need about 115°F or more to kick out much heat.

The go into the floor like this. Since it is a forced air convector you get a lot of BTU/hr output compared to fin tube or panel radiators.

you could add a vented wall furnace or console heater if you want it to be a separate system. there are versions that can vent out the sidewall.

electric heat could make sense, it will cost more to operate but the lower installation cost might offset that.

a small mod con boiler and panel radiators is another option.

if you pick the right wall furnace or console heater you could get a model that will run off a thermopile or built in hydraulic thermostat and will run in a power outage.

I'm with @hot _rod on this one. I have seen a project where the floor heat was sufficient for the home where there was average (35% of the walls are windows) glass, but the "solarium" with 18ft ceilings and 18ft floor to ceiling glass wall would not get enough heat after the outdoor temperature dropped below 40°. That same boiler would produce 180° water to run through Runtle® radiant panel baseboard radiators that wrapped around the perimeter of the glass walled room.   That was able to heat the room when the temperature dropped to 15°.  But when the outdoor temperature reached below that, the warm air duct system would operate to take up the slack.  

Since you are using minispit systems for your cooling, that eliminates the duct coil option.  So perhaps the kickspace heaters as the third line of defense is the best idea.  That can all be operated from the same boiler that most likely has extra capacity that the floor heat as a heat emitter can't use.  

As far as the minisplit heat pump is concerned, I don't see how that will be of any benefit.  In understanding the concept of a heat pump you need to  think of how it works.  Picture a window air conditioner installed in a window backwards.  The hot air blows inside while the cold air blows outside.  That is a great concept in the right climate zone. If the outdoor temperature is high enough the compressor can take the outside air and remove some heat from it, compress it into a hot gas in the refrigerant and let that heated gas into the indoor coil so heat can blow into your home.  

So the heat from outside is compressed and sent inside.   As the outdoor temperature gets lower, there is less heat to compress so the compressor is doing the same amount of work and producing less and less heat as the temperature gets colder and colder.  Those low temperature heat pumps are just larger heat pumps that have more capacity so it will work at a little lower outdoor temperature.  You can't make more heat from air that is -20° by just saying it is a low temperature heat pump.  You need to use larger coils and compressors to do more work to make enough heat from that colder outdoor air.  I also find them to be less reliable than standard heating equipment as the temperature drops.  

If the heater you have has additional capacity, why spend more $$$ on another heat source?  Just use better heat emitters to get that heat into the room(s). 

In answer to"

“Can the in the floor convectors be added to the current zone plumbing, or do they need a new dedicated supply, return and circulator? My current boiler temp is 130*f”

By all standard rule of thumb calculations and some of the more specific actual engineering calculations, a 3/4" pipe can deliver 40,000 BTUh when there is a 20° temperature difference and a flow rate of 4 gallons per minute. So the question you need to answer is "does the problem space require more than 40,000 BTU of heat per hour?" To be entirely sure, I might put the kick space heater(s) on a separate higher temperature circuit in order to get the maximum heating output from them.

As far as an auxiliary separate heat source is concerned, Electric heat as a backup is the most reliable since there are very few moving parts involved.  Set the thermostat at 55° on the standard heating system when you are away,  set the electric heat at 50° all the time.  When you are away, if the boiler heat fails, the electric will keep the place from freezing up. 

@ColdHouse : "I would really like to have a completely separate back-up heating source, separate from the current main heat."

What do you really mean by "completely separate"? What contingency are you guarding against? Mechanical failure? Water leak? Propane leak? Running out of propane? Losing electricity for a prolonged period? What about if a storm blows a branch off of a tree into a house and it breaks a window? What if a burglar breaks in?

You can always think of a situation where the heat fails. There is no technology where a house can be left completely unattended in the winter with 100% certainty that it will remain heated.