FreeBSD and macOS have a dc implementation written by Gavin Howard which
accepts the -x option for extended register support.  In this mode, if
the character specified for a register is a non-newline whitespace, it
can be followed by text matching the regexp `[a-z][a-z0-9_]*`, which is
assumed to be an extended register name.  This allows for the utility
names more like bc identifiers and therefore a wider variety of
functions.

Would it be possible to add such a mode in dc?  Providing I have the
time, I'm willing to write patches to implement this solution if
upstream or Debian will accept them without a copyright assignment, but
if someone else would prefer to write them, I am of course willing to
let them do so.