It sounds like a fabulous potential setup, whatever you decide to put in there.
Unfortunately, given my experience with
Heteractis magnifica at least, I would venture that 24" square is not really going house that anemone comfortably (as you have already speculated yourself). However I do think that a 24" tank can make an ideal habitat for several other species, depending on aquascaping options; including (as already suggested)
Entacmaea quadricolor,
Macrodactyla doreensis,
Heteractis crispa,
Heteractis malu. Also quite possibly good habitat for carpets, depending on the size of the carpet that is. Some carpets can also get quite large,
Stichodactyla haddoni and
S. mertensii tend to grow towards fairly large diameters. You did mention you weren't too keen on carpets, so I guess I'm just throwing that info in for the heck of it. (They're my favourite so you have to put up with me talking about them
)
Collection impact and ethics notwithstanding, I think that
if one were to attempt an anemone, that by designing the system around the needs of the anemone first, is certainly commendable. Keeping a dedicated habitat for an anemone, I think, can be an interesting and fun challenge.