So, you're wrong about it being a 3V (or close) chip, 3V chip is the XC9536XL, this is a 5V compatible chip that needs 5V when programmed - there's no hidden regulator in the image I posted, I have built four of those myself (image doesn't show my build), it's just the one chip.
You usually keep to facts but this time you're just making things up.
A parallel JTAG programmer cost me €11 with shipping in 2012, I believe it was cheaper than an eprom programmer, but that's true a lot of low cost programmers can't program it. A lot of microwave ovens can't program it either as they weren't made to do that...
I have used my JTAG programmer to program that chip in the Nintendo 64 RGB DAC that "viletim" made as well and a few other things.
The first XC9536 chip I found on eBay now cost $4, I don't care enough to compare with what a "proper" eprom costs. Difference is you don't have to experiment with capacitors and nobody seems to have a problem (compared to eproms) with using this chip as a replacement.
http://www.ebay.com/itm/370838187852
I think the 5ns chip were too fast though, so don't buy this particular one. In the comments at nightfallcrew we discussed speeds a little.
Here's a better price for 50 pcs:
http://www.ebay.com/itm/380932198267
So if you made 50 of these to sell, including the cost of the programmer - perhaps not overly expensive.
How old motherboard do I need to try? I'm not sure I'm up for desoldering PLA:s just to show that it works, I really don't care to prove it as I'm not selling the product.
If programming slows down needed outputs -what's the problem with that, isn't that what you're supposed to do with a programmable chip - adapt it to your needs?
Just like people to know there's another type of replacement than an eprom - that you can also make yourself. If you just need one then a SuperPLA might be cheaper solution - or why not sell the parts again after you're done?
EDIT:
If you happen to have an eprom programmer and need one M27C512-90B6, the cheapest one I found on eBay was $7 with shipping (about $3 without). But why not buy a ready made one from Italy, only €16 plus shipping - that's right you can't just plug the "$1" eprom in, it needs an adapter as well...
http://www.ebay.com/itm/171756619191
Did anyone want to specifically put the cheapest things they can find in their C64, not the best working? You can still buy 10pcs of M27C512-90B6 for $20.83 so why start experimenting with another eprom and diodes and resistors...?