Ok, the best that I have come up with is to use:
auto ACS_ULCORNER()()
{ return acs_map[cast(ubyte)'l']; }
which you can call using addch(ACS_ULCORNER());
There is an extra set of '()' that aren't there in the C version, but I
haven't come up with anything that would allow me to remove them...
Thanks for your help.