I think Juicer had the coolest looking design. The batteries ( jumbos too, far more capacity than the little cells that most packs are built from) are arranged to look like the cylinders of a Vee engine. Hiding in plain sight, accessible, but gets that weight down nice and low.
I believe Kosynier packed their battery into the faux tank of their "investor bait prototypes", sort of like you suggest, but there's a LOT more space in a motorbike style frame than in the crescent moon of a cantilever. Their dummy cylinders are plastic moldings, or will be if they ever get them into production. After something like 5 years of waiting, their investors have got to be getting antsy, I would....
I've seen quite a few boardtrack racer designs that take this path. The drawback is that it puts that considerable weight up very high. This is the Michael Blast "Greaser" that Bike Berry sells
Moped styled bikes have a lot of options, ... they can put batteries in a faux tank, or on a rack over the rear wheel, or anywhere inside the frame space, which is mostly open. If you start out drawing the wheels, the seat and the crank, then the motor, and draw in the frame with batteries last, ... a lot of options open up. Like this long range setup, that carries it's battery weight nice and low:
Sondors built a custom triangular pack to fit the space, rather than glob a standard pack on somewhere:
I think I could do this all day. The field of e-bike design is rapidly expanding and lots of interesting new designs are popping up every week.
A quick and dirty way to get up and running fast, is to put the battery pack in a duffle bag on the rear rack. The weight is high and outside, but it's cheap and easy to swap or carry inside for recharging and theft safety. Personally, I'd give some considerable thought to function over form before committing to a laborious build.