I was under the impression that the Blunami system was just a DCC Decoder, not a battery powered system. however looking deeper it seems you could rig it up that way in partnership with BlueRail and some of their products
since the blunami chip from soundtraxx is bascially a DCC Decoder, and a speaker to go with it. that pretty much takes up the tender of the locomotive for space. which means that any batteries would have to be wired into an auxillary car, or be small enough to also fit within the tender. In my experience, there is not a ton of extra room for batteries, or easy access to charge them, in most HOn3 Tenders.
While it is probably doable, with the tinniest of batteries available and a tender shell modified to maximize interior space, I think the install would be incredibly finicky, and you may wind up with an install so tight that simply charging the batteries would become a chore.
From a saving the knees perspective, is it possible to get all of your track laid, flip the benchwork over, and wire with the underside facing upward to save crawling around on the floor? Lay track, punch feeders through and solder to rail, then flip the whole thing over and tie into your main bus wires?