I am not sure if you can check-in at a different airport, but I know you can change seats/reprint BPs from a different airport once already checked in.

I was flying BD BRU-LHR connecting to UA LHR-SFO. Checked in with BD all the way through and received both the BRU-LHR and LHR-SFO BPs, however, the seat assignment for LHR-SFO was different from the one I reserved (E- as opposed to E+).

At BRU, I went to the UA ticket purchase desk (UA check in was closed) and asked to change my seat back for LHR-SFO. Seat was easily changed and new boarding pass printed at BRU for a LHR-SFO flight.

So perhaps the OP can do OLCI and have the ticket counter at BKK reissue the BP for SIN-NRT?