Is it possible to combine an amex 241 with non-ba metal.

For example: I want to book LHR-SIN in CW. No availability on date I want but there is availability LHR-KUL and then a connecting flight KUL-SIN in Business. This shows as available on the Ba site when I say don't use my companion voucher but then when I select companion voucher it isn't available as the KUL-SIN is on MH. If I speak to the call centre can I use my Amex 241 on the LHR-KUL but then book a connecting KUL-SIN using Avios (not using my 241) all on the same PNR but so that I don't have to clear customs, re-check baggage etc.

TIA