Well, it's your frame and your paint gun and your angle grinder and everything so if that's what you want to do, it's not something I would do, but, yeah, go for it.

Something you might consider if you want a decent English fixed gear bike with a recongizable name is a Bob Jackson track frame. Current price seems to be about 413 BP, or about $520 at today's exchange rate. You could sell that Mercian for a nice bit of change to someone who wants to set it up for IGH(no hanger) and have the Jackson for probably less than a decent paint job.

You make a good point, I think, about people doing modifications to old bikes like adding brazed fittings for canti brakes, adding bottle bosses, etc. I'm having trouble justifying approval for one without approving of going the other way. Still, I wouldn't do it, not to a Mercian.