What is a safe power rating (W-RMS) for the step-up transformers? I know this depends on panel size, but I plan to make that decision based on the availability of transformers.
Will 10 Watts-RMS do the job for standard size panels?
There doesn't seem to be any uniform standard for quoting power ratings of audio transformers. If they quote the power rating at 100 Hz, the number will be high and if they quote it at 20 Hz it will be low. The marketing dept usually takes over and says"gee, we can get more money for a 50W transformer than a 20W transformer, so we'll call it a 50W transformer by measuring the power at 100 Hz instead of at 20 Hz". If your transformers are rated for 10W at 20 or 30 Hz, they're probably fine. If they are rated for 10W at 100 Hz, you may have problems.
I use a pair of 15W (at 30 Hz) transformers for each of my ESLs and they never complain, even when driven by a 200W/ch amp. I have used them with one transformer per speaker, and they likewise never complained.
FWIW, the Acoustat 121 MkII interfaces use an xfmr for the "LF" that has a core (from memory) of about 4 x 4 x 2.5" thick? While the "HF" (which covers down to <200Hz) is about 3 x 3 x 1.5"...
There's not much "power" being drawn, since the load is capacitive... but there is some requirement for voltage breakdown, especially to ground and to the primary side... but most "output" transformers are likely to be sufficiently well insulated.
You could always opt for the homebrew pressure cooker vacuum varnish impregnation method to boost that up, if ur worried about it.