If you have the time, as you can refuel from water, all you'd need is a bypass on the output of your scoops/fueling pumps to (instead of sending them to inboard fuel tank storage) send to tanks for Hydrogen in the cargo hold.
You'd pay some time at fueling time on the planet or at the ice asteroid, but you'd spend less time between jumps so it's probably a net plus. Plus then you only need to store LH in the tanks - the O2 is emitted out of the ship by the purifiers.
So fill up your tank, take the time to purify enough to fill your new inboard stowage vessel in the cargo hold, and off you go.
And:
1) To me this is entirely what an interior demountable tank or collapsable tank is. So you already probably have rules to pay for it.
2) Your non-baffled and lattice-filled cell cargo bay tank will behave differently than your main baffled and lattice-filled main fuel tank:
a) Your cargo bay gets a hit - bad for the tank potentially, then perhaps bad in that you get a follow on fire or explosion unless the ship's cargo deck is vented to vacuum either by the hit or as a battle or accident preparation step and bad for your fuel supply because you'll likely lose all of it in this instance.
b) Some too amazing maneuvers may result in the more temporary style of tanks showing wave surging and/or brackets coming loose and then the tank moves, gets punctured, and same as above. I assume a collapsible tank may in fact have a fair degree of tear or puncture resistance and may have some pretty good hold downs, but if you are holding 52 tonnes and you jink and juke or buffet during ducking into a GG (or similar), then maybe those ties might have to make some tests.
That's simpler than the cracking of water in the cargo deck and then trying to get back both reagents and meanwhile carrying huge weights of water.
The only counter argument could be:
a) Load water
b) Start run for Jump Point, start cracking
c) Jump
d) You probably want the Hydrogen Separator (let's call it that) could probably process water faster to crack loose the LH, but it needs to purify maybe too, so let's say it produces 10 dTons (processing roughly 100dTons of water) in a day. The purification is actually simpler than what comes from a GG so that's why can be managed. It might be costed as 1 dTon (though you gang units) and 1.5x the cost of 1 dTon of fuel purification. It outputs O2 on one line (to be vented or stored - better vented as Liquid O2 can be dangerous) and LH on the other output line to go into main tank, drop tank, or nearby demountable or collapsible tank (the latter three purchased separately, the first is built into the ship!) and by the end of your jump (actually 5 days in), you've got your 52 dTons of LH in your collapsible tank in the cargo hold.
e) If you have enough cargo space to embark all that water, while you figure out jump coords, you pump the fuel in the collapsible tank over to the main fuel tank. Then go back to b).
This particular version might be faster than process-at-site but it also means crapping up more of your cargo bay.
So my two summarized ideas:
Method 1): Stay extra long at fueling location and have the purifiers purify into main drive then collapsible tank in the hold to allow rapid follow-on Jumps
Method 2) Stay a little longer at the fueling location, fill a large water bladder with water, lift off, and let the Hydrogen Separator go to work. by the time you reach Jump exit, you'll have a full hydrogen load available to jump again. Rinse repeat until you need more water.
TomB