Ok well remote battery holders is just a hack the kits with 2 battery dummies to place in a holder is just overpriced and funny.
Nvram is a good idea but price wise isnt overly practical, A new board mounted battery holder with batteries reguardless of some locking technology is still rolling the dice would $250 even replace the MPU? and if you have a meltdown and why worry about fixing acid damage if it can be avoided.?

Honestly the ultracap mod is the most practical no wired hanging off the board and realy about the same price as a remote holder hack. Ultracaps work same as the originals in 90 and earlier machines for sure havnt converted a WPC with one yet so cant confirm results yet
But heres a pic of the modded sys11 mpu. cost wise about $5-$6 can be mounted on the backside of the board and a battery holder can be mounted on top to look original and even mounted this way replace the diode and remove cap to convert easily to original if that trips your trigger.