I looked for a PPM for 64-bit 5.10 but none is available

Yes, the first perl that built with MinGW64 straight out of the box was 5.12 - so, since I'm using the MinGW64 compiler, I've currently limited myself to building only for 5.12 (as regards x64). When 5.14 is released I'll cover it for both x86 and x64, too ... so you might be able to catch up then :-)

I can build these ppm's for x64 5.10 and 5.8 (I think) - and would happily do so for anyone that needs them, but by the time the x64 builds of ActivePerl become prevalent I expect that 5.10 and 5.8 will have long been superseded. I therefore don't currently intend to build and upload ppm's for these earlier x64 builds.

I've thought about uploading the libraries themselves. But even if they were available, you'd have to grab the MinGW64 compiler (and make some modifications to a couple of your ActivePerl .pm files) before you'd be able to run the Inline::C script I posted. (When I switch to Math::GMPz, there's about a 6x slowdown - mainly, I think, because perl's for loops are much slower than C's.)
Besides, it looks to me that you already have far better solutions ;-)

I think the transition from 5.10 to 5.12 happened much to quickly

It was certainly pretty quick - and 5.14 is due out next month !!

Cheers,
Rob