Substitute र + ◌ृ by "reph + ऋ" without introducing ligature glyphs.

Ra-VowelSignVocalicR

@vocalic_liquid_matra = [
  dvRvocalsign
  dvRRvocalsign
  dvLvocalsign
  dvLLvocalsign
];

lookup flip_ra_to_vocalic_liquid {
  sub dvRa' dvRvocalsign  by dvRvocal;
  sub dvRa' dvRRvocalsign by dvRRvocal;
  sub dvRa' dvLvocalsign  by dvLvocal;
  sub dvRa' dvLLvocalsign by dvLLvocal;
} flip_ra_to_vocalic_liquid;

lookup flip_vocalic_liquid_matra_to_reph {
  sub @vocalic_liquid_matra by dvReph;
} flip_vocalic_liquid_matra_to_reph;

feature blws {
  sub
    dvRa'
    lookup flip_ra_to_vocalic_liquid
    @vocalic_liquid_matra'
    lookup flip_vocalic_liquid_matra_to_reph;
} blws;

# Then you can continue to position the reph.
# Core Text (OS X) doesn't apply `abvs` (say, reph + anusvara) to this reph properly,
#   so a duplicated `sub dvReph dvAnusvara by dvReph_Anusvara;` for it might be necessary,
#   which makes this hack even more dirty... XD