Modern capacitors have made the separate energy storage unit mostly obsolete. That's why one doesn't see them made anymore.
DAUs were invented before the advent of the mini super-capacitors now used to power stand-lights in modern dynamo powered bicycle lights. The lead acid batteries used in the units had the same problem as all old lead acid batteries, in that the constant current recharging them eventually causes the batteries to corrode to the point of leaking and developing memory issues.
FSU units came into being when Sturmey Archer got sick of dealing with leaking battery issues and couldn't obtain a better energy storage solution at the time. Thus the switch to a filter switch, where the stand-light was powered by conventional batteries (like a flashlight) whenever the dynamo current was detected to have cut out.
The DBU units are essentially a flashlight setup akin to modern battery powered lights. At the time it would been prohibitive to stick D batteries into both the head and tail lights for such low input, therefore the DAU/FSU battery tube was reused with a simple switch through the headlight. Though I must say that I have encountered one British Miller head lamp from the late 1950s which used a large EverReady 9 or 12? volt flashlight battery with a remotely wired push button on/off switch.
Sanyo Eneloop batteries should have better longevity than the original lead acid batteries in the DAU/FSU units and never leak. However, since they are 1.2-1.5volts vs. 1.5-2volts of the original batteries -the battery powered stand-light illumination will never be quite as bright as the original bulbs or the maximum potential illumination from modern LED bulbs. Unless of course someone can create a custom battery pack or protected capacitor (the capacitor could be potentially lethal if poorly designed...hence my tendency to want to stay with protected circuit rechargeable batteries) to fit inside the tubes with the proper connections.