Not that the bolt on cog isn't a great idea, but I don't think it would work (at least not very well) for what the OP has in mind. Using it on the disk side of a multi-speed hub would require two dramatically different length chains. It is possible to just have two quick links, but this is getting to be an increasingly difficult 'flip'.

Flip flop IGH is complicated by cable routing, when it's in 'fixed mode' your going to have the shifter cables just floating around, not attached to anything.

I guess what the OP wants is a shiftable flip flop hub with 75%, direct drive, 125% and fixed gears. The S3x just came out, can't we be happy with that for now?