The pressurized water ballast system would work, but it does need a larger volume tank than some other systems, as it only uses 50-75% of the tank volume. However if you were willing to switch battery technology to lipo or better still Lithium Life PO4, that would reduce the space needed for the battery pack by approximately half, and this would free up space for a larger tank. Also bear in mind that the engel tanks are quite slim (about 75mm diameter). You can fit a wider tank in, which will increase volume a lot.
I'm working on Roy's smashed U47 at the moment, although not right in front of me at present, but I think the inside bore of the cylinder is 10cm isn't it? Will have to measure it again. But assuming it is, a piece of 10cm acrylic pipe 23cm long will give you about 825 litre displacement with a tank 50% filled, and well over a litre 75% filled.
Another possibility if you don't mind retaining a bit of positive bouyancy i.e. top of sail just above the water, is a vented tank system. Would be very inexpensive (e.g. under £30), and that should work very well. You could use a Kavan (Ripmax) geared type pump (2L per minute) to pump in and out, no pinch valve required becuase you're not pumping against any pressure. That should give you a dive time of about 20-25 seconds. Not ultra fast but reasonable. The other way would be to use a centrifugal pump just to pump the water out, or blow it out with a diaphragm based air pump. Both of these systems would require the tank to flood using gravity as these only pump in one direction, so you need a large opening at the bottom of tank to minimise resistance. Might be a bit fiddly to incorporate that into the Robbe design.