>>I have an Ernie Ball volume pedal with a 250k pot, but obviously it is too large a value for the DL-4.....<<
have you tried it? the only thing I had to do to an expression pedal to make it work with a DL4 was reverse the tip & ring connections. I didn't even look at the range of the pot itself.
I'm pretty sure that all the DL4 is looking for is a voltage; the socket will supply (say, for the sake of argument) 5 volts out of this socket, & expect a proportion of this to be returned to it by way of a control signal. the amount of resistance the pot introduces is insignificant; the part of the DL4 that measures the returned voltage probably has a very high input impedance & so very little current will flow.
but I could be wrong..... :-)
in any case, so long as the ernie ball pedal is wired correctly, you won't hurt anything.