A GEN_RID is an OID plus some opaque data. It's up to an application to understand an OID and interpret its data. The value of a GEN_RID can be as simple as an int or UTF-8 strings or as complex as a nested ASN.1 struct for Kerberos principals.
I have modified Lib/test/make_ssl_certs.py to include two GEN_RIDS:
otherName.1 = 1.2.3.4;UTF8:some other identifier
otherName.2 = 1.3.6.1.5.2.2;SEQUENCE:princ_name
[princ_name]
realm = EXP:0, GeneralString:KERBEROS.REALM
principal_name = EXP:1, SEQUENCE:principal_seq
[principal_seq]
name_type = EXP:0, INTEGER:1
name_string = EXP:1, SEQUENCE:principals
[principals]
princ1 = GeneralString:username
1.3.6.1.5.2.2 is the OID for Kerberos public key init (pkinit), used for e.g. FAST pre-auth and SmartCard authentication.