If I have a spare moment, I'll add the clear value as an optional second parameter, as you suggest. I guess it's fair enough that 0 is not always the best value (e.g. &FF might be better for unused bits of EPROM code).
Otherwise, for the moment, just use a loop - or the following macros!
Code:
MACRO FILL num, value
IF num>0
FOR n, 1, num
EQUB value
NEXT
ENDIF
ENDMACRO
MACRO FILLTO address, value
num = address-P%
IF num<0
ERROR "FILLTO address is a backwards reference"
ELSE
FILL num, value
ENDIF
ENDMACRO