Bart, I'm not sure if it helps but I just put a couple Del Tang units in a few loco's. You might want to check my post.
I used the Del Tang Rx63 in Bachmann Porters and there was enough room for batteries too. It was a challenge to find the right cells to fit but in the end I worked it through. Placing everything in the cab allows me to run without the need for a tender.
I also put a Rx 61 unit in a climax. For batteries i opted for Nimh. I found them through ebay. 1/2 and 1/3 AAA cells that fit. I made 5 and 6 cell packs.
Everything seems reliable.
If you are looking to hide electronics in the boiler, it might take a bit of finesse, but I would imagine not impossible. The difficulty is that some of these loco's have so much detail that taking them apart to insert electronics is super tedious.