I’ve tested and found a Malkoff MDC 2v-3.2v head on an L1 Lumamax will fit, and function this way. The L1s have the driver in the body instead of the head, unlike all of the SF two-stage lights that came after it. The Malkoff P60 drop-ins will not function on the L1, however, because they use a different type of regulation. Both high and low will be the same. The MDCs do not have the same regulation (or any at all, which is why its runtime curve includes diminishing output), so this unlikely combo works, probably with a downside of additional losses from running two drivers.
As far as custom mods... the multi mode feature of the Surefire two-stage tailcaps like the EDCL-*T seems to be more than just the resistors in the tailcap; the driver is amplifying the effect. You can see what this looks like by direct driving an LED with a two stage tailcap in line. You may find the low and high modes to be too close together to be useful, because the low mode resistor's value is not high enough to significantly bring down the current to the LED. Without knowing the Surefire circuit design, I would guess the driver is somehow using it as a signal to select mode.
The regulation circuits of the custom drivers I’ve tested so far have canceled even the minimal effect of the tailcap resistor out, making what would be the low mode the same as high. I am not aware of anything other than a Surefire driver that can do this-- so a mod would require ripping out the driver of a two-stage SF light, inserting it into your target light, and finding a way to re-thread your target light to accept a SF two-stage tailcap. A lot of custom work for little payoff, IMO.
The L1 heads have no driver so they're prime hosts for this type of mod, you need only replace the LED in the head, or find another E threaded head with no driver and use that.