looks like avpops module documentation does not allow pseudo variables as avp names although, for example, avp_pushto function does support them.
1.5.7. avp_pushto(destination,name) ...
name - which AVP(s) should be pushed into the SIP message. Parameter syntax is:
* name = ( avp_name | avp_alias )['/'flags] * flags = 'g'
and
1.3. AVP naming format
The format of the parameters specifying an AVP in functions exported by this module is: $avp(avp_flags:avp_name) or $avp(avp_alias).
according to above, for example.
avp_pushto("$ru/domain", "$fd");
should not work, but it does (at least in 1.1.0).
-- juha