Typical 18650 battery holder is 3" x 13/16" x 5/8" (3/4" with battery installed). Obsidian card dimensions: 2-3/16 x 1" x 3/8".
If you note the last dimensions I stated here 3/4 & 3/8. If both are added together, it comes out to 1-1/8". Since US uses a 28mm speaker (with holder)for their sound cards, it's POSSIBLE, to use the larger 18650. But, since the battery can't drive the voltage needed, as stated by others, you would have to add a voltage booster to push the output to 7.4v.
Before I go any further; I had been looking at this for my own uses in making a plug in modular chassis system to make hot-swappable LED modules & battery chassis because I HATE, HATE, HATE the loose wires inside of hilts. I abandoned this project because I just could not make it work with my limited resources................and this was using 2 empty hilts: Aeon v4 & Initiate v4.
Thanks for including the physical dimensions in your post, it certainly helped to add some perspective. It certainly makes sense when considered with the soundboard that US use the 14500 batteries, however like I said originally it is much easier to find good quality 18650 batteries even in the UK where I live. Since I've had my Dominix V3 LE I've been through five or six sets of 14500's two sets of which would never recharge and one set that never took a full charge. Where as my flashlight which honestly I've had as long as my saber and use way more than my saber is still on it's first set of 18650's one in it and one spare.
I certainly agree with you on the loose wires, they are somewhat of an irritation. Now I run my Dominix without the sound card as at the time I could not afford it, that said I think you might be onto something with the modular chassis idea.