If you're doing the (larger) hydrogen tank, how 100% sure are you that you're not making an explosive fuel-air mixture, given that hydrogen burns at just a couple percentage concentration?
Well, for the actual system they're considering, it's the larger (volumetrically) part of a single tank, the LH2 and LOX being separated by a "fibreglass honeycomb", which I guess is glued in place after welding together the bottom end of the tank and the main length of the tank body, but before welding on the top end. I'm not sure whether the LOX is in the upper or lower part of the tank.
Unlike most people here (and I think you and I have discussed this before), I've worked with hydrogen gas too (in analytical instruments), so I appreciate what you're saying about it's flammable limits. What I'm wondering is - with relatively minor modifications to the tank's structure, would it be workable to get rid of the dregs of the fuel by pumping O2(g) into the H2(g) headspace of the tank and lighting a flame of O2 burning in hydrogen. So you approach the UEL of [H2] from the other end, and you have your oxygen going into a mix of H2(g) and H2O(g). Any water that condenses out is uninteresting from an explosion safety point of view. For maintaining the desired pressures in the tank, you'd need an inert pressurising gas - N2 being where I'd start from (might need He for thermal reasons ; meh).
Plan B might be to install a mixing fan into the H2 tank which was covered in Pt-coated wire wool, and to use that to agitate the gaseous contents of the H2 tank, again while bleeding in O2. Monitor tank temperature (Pt-resistance wire as in a kathaometer, perhaps ; kill two stones with one bird?) as the Pt assists the reaction ; if it gets too warm, turn off the O2 supply and let some of the water condense out. Again, make efforts to avoid ever having an atmosphere in the tank that isn't a mix of H2(g) and H2O(g).
I bet the procedures for filling a newly-made tank with it's first charge of LH2 are a real fun read. I don't recall anything about it in "Ignition!" - but that concentrated more on exciting fuels like red-fuming nitric and the "mythical monoprop" and less on dull, pedestrian fuels like LH2/LOX.
I'm tempted to change my signature to :
'monopropellant' is an anagram of 'explosive' for all important meanings of "Kaboom!"