As a thought what about a super cap and removal of D17 for the battery replacement? About the same cost as batteries and a long time maybe more elegant solution. It seems popular in the Bally world, but not in WMS which I don't understand.
I do like the idea of keeping the board nice and tidy. Will definitely look into this for future. Are there any drawbacks to these capacitors?
I know that you have to power them up regularly to maintain the charge but that's hardly a drawback. 
Super cap issues, as I see them;
- at 5.5V they are slightly higher voltage than 3 x 1.5V alkaline cells but 5101 spec sheet says this is ok (good for 7V)
- Loss of originality if this is an issue to you or subsequent owner
- removal of D17 may cause some grief if someone reverts to batteries at a latter date and doesn't spot this
- harder to reset to factory defaults - have to use dips rather than just remove battery
- said to hold charge for about a month
- I did read somewhere that they can leak - I've not seen this, but could remote mount
- I wonder if it causes an issue when doing work on the board and remove the 5101 as it will be powered unless you remove the cap? maybe an argument to remote mount?
I have a Flash I'm working on at the moment and plan to do all the board work so I plan to try it on this and will report.
A supercap is the same as any other capacitor when it comes to the voltage rating. Most are rated at "5.5V" - meaning the MAXIMUM voltage that may be applied is 5.5V. Look at ordinary electro caps, you happily fit a 63V rated cap in place of a 16V rated one. Same story here - the Supercap will only charge to the voltage applied to it, if you apply 3.6V then that is what it will charge to.
