Felt?...as a leveler....not sure I'd go that route ....in fact, I've never heard of such.
The self leveling, Seems to me if it lasts over luan and under vinyl like it does why wouldn't it last over ply.
I still think your best bet is to go with the sistering method.
If it was built in 72, yes it's platform framing, but how did they come up with 24oc, that has me baffled.
One more thing, better contact the tile manufacturer, I know 16oc requires 1 1/4 sub-strat but 24oc, well like I said better give em a call.