Difficulty implementing validation of user input (getting RSPLS_CR013)

We have a business requirement where the user creates new characteristic combinations (in the manual planning using AO 2.2) entering values for IOBJs ACCOUNT and CUSTOMER.

The following validation of the input of the user is required: if selected ACCOUNT ends with 1 then CUSTOMER must be filled (i.e. <>'#').

We have tried to implement this through IF_RSPLS_CR_METHODS~CHECK in ZCL_RSPLS_CR_EXIT_BASE, but get the error RSPLS_CR013.

Please note: this is despite ACCOUNT and CUSTOMER NOT being marked as 'access mode for result values' based on characteristic relationships. They are both marked as 'based on Master Data'.

Hence, the system requires that the valid combinations are generated in IF_RSPLS_CR_METHODS~CREATE, right??? We are reluctant to use this since it means that we would have to create the cartesian product of ACCOUNTs ending with 1 and CUSTOMER, which would be quite a LOT of combinations :-(

Another alternative we have looked into is to perform the validation using a FOX. This we have discarded since only message type E prevents invalid combinations from being saved, which in AO requires the user to log in again.

Summary: is the generation of the cartesian product of ACCOUNTs ending with 1 and CUSTOMER our only option for the simple check 'if selected ACCOUNT ends with 1 then CUSTOMER must be filled' or can someone suggest an alternative ???

We are on Embedded BI HANA 750 SP3 using BPC.