I wonder how efficient it would be to have a 2.5 or 3 gallon electric WH at the farthest bathroom where you want quick hot water, and plumb its inlet with hot water from the main WH?
Or a relatively low-wattage tankless; once the inlet water temperature exceeds the set temp, it's not going to be doing anything anyway. Plus, what I've found frustrating in a few places is just wanting some hot water to wash my hands, and having to wait 2-3 times as long as the shower or tub for the sink faucet to be able to clear the line.
Of course, sizing is tricky, since Fed regs limit the shower head to 2.5GPM at 80PSI. (At time of sale, of course. If you've drilled out or "accidentally lost" the constricting bit then you're just going to have to measure.) I'm not sure about city supplies, but I know our balance tanks were usually set somewhat under 50PSI. (According to popular Mechanics, 40-50PSI is normal for residential customer on a municipal system.) How that affects the flow rate depends on several factors, but the easiest way to figure it out is a good old fashioned bucket of known volume and stopwatch.
It also comes down to what you're willing to accept for the delay time; if you're satisfied with just knocking the chill off until the main supply gets there, one of the 110V <=15A heaters may be enough, and won't require any rewiring, (Some of these are actually mini-tank, but may be more space-efficient than the typical cylindrical tank mini heaters.) or some 15-30A ones use two circuits, so you're still only dealing with normal 110V wiring.
YMMV, though; the only sizing info I can find on them is "x GPM @ 105 from y inlet temp," whereas they will, of course, allow a higher flow rate and heat from a lower inlet temp, but just won't get the water all the way to 105F. IME, 98F is not uncomfortable regardless of ambient air temp, so you can easily stand a 7F drop there without standing around shivering until the main hot water shows up...with a bonus of not dropping below that if the main heater runs out while you're showering.