This is the current workaround as I understand it:
Convert to CPL(H) - requires POF(H) plus necessary flying training.
When you have enough hours to get ATP (not forgetting type rating on large helicopter), claim an exemption for the IR(H) based on the fact you already have the ATPL(A) Theoretical Knowledge, and presumably multi-crew. This works because the only difference between the CPL(H) and ATPL(H) TK is the IR. It doesn't work the other way because the fixed wing CPL is of a lower academic standard.
phil