Daryn Skinner

I'd say go for it. The "Critical Mass" issue is really of no issue at all. Planted tanks are popular for this same reason. More plants, less filtration/maintenance that is needed.
There will be a equilibrium but there will be some experimenting to do to find it. For instance, if you use only spider planes and pathos only, they are not heavy feeders. They about survive forever in tap water and not much else. Plant something that fruits and or flowers and they will require more food. (i.e. strawberries, tomatoes)
Also the bacteria in the substrate will eat up some of the plant food. It will try to always achieve the equilibrium. If this does not leave enough for the plants just get rid of the substrate. That will also make cleaning easier.
Good luck. And post some pics to keep us updated. I'd love​ to see how this comes out.

Glenn Herbert wrote:It is always considered a very bad idea, and prohibited by code, to connect two appliances to one chimney. Draft can be unreliable, and one may have a bad effect on the functioning of the other.

Not true, is done all the time. Think your typical gas furnace and water heater.
The catch is, there is a formula to figure the size of the flue that's needed. It uses known figures like BTU output, exaust temps, length or run, Ect... So if you can figure those out, it's totally possible to do and know if you can do it safely.

Baxter Tidwell wrote:

Fresh air is no problem, unfortunately. The shop is three 40-ft steel shipping containers welded together. It is very difficult to get everything air-tight, especially around doors and windows. There's always a draft somewhere.

But that's not a problem because it's a workshop, and you'd expect it to be drafty. Maybe this winter I'll want to seal it up a bit more if it's uncomfortable. If that causes a constriction in airflow, perhaps I can punch a hole in the outside wall next to the heater core and let in some fresh air. I'll figure it out as I go along.

Baxter, a couple of things for ya,
Maybe you've already done it, but putting in a dedicated fresh air intake (in HVAC they call it combustion air) is a great idea for a couple of reasons.
(I'm actually in the HVAC service industry, so I'm kindof a traitor for being here 😛 but this stuff is fascinating)
1. Being that your shop is already drafty, when you fire that thing up it's got to pull that fresh, cold air thru the cracks and crevices and thru your shop to get to the RMH, further chilling your shop along the way.
2. Your heater will be more efficient for 2 reasons. First reason going back to #1. Your not heating the "combustion air" as it travels thru the shop.
#2 colder air causes a more efficient burn. I'm not real sure of the science behind this, but it's something about denser air, more available oxygen, blaah, blaah, blaah.....

But yea, in the drafty shop, if it's possible put one in. Make it as close to the intake of the RMH as possible, if not directly into it sealed up and everything.