I did just this last week on SQ, interlining to AA when checking in at Jakarta. The AA flight was booked on a separate itinerary through AA. I provided the AA flight #, ticket # and record locator, and it literally only took the Indonesian check-in staff 30 seconds to do this (I was bracing for a problem).
Here is a list of all airlines with whom SQ has an interline agreement, courtesy of KVS:
AA - AC - AE - AF - AH - AI - AM - AP - AS - AT - AV - AY - AZ - A3 - BA - BD - BE - BG - BI - BP - BR - BT - B6 - B7 - CA - CI - CM - CO - CX - CY - CZ - DJ - DL - EI - EK - ET - EY - FI - FJ - FM - FV - F9 - GA - GE - GF - G3 - HM - HU - HX - IB - IC - IG - IR - IT - IY - JJ - JL - JO - JP - JU - KA - KC - KE - KF - KK - KL - KM - KQ - KU - LA - LG - LH - LO - LP - LR - LX - LY - ME - MF - MH - MI - MK - MS - MU - NF - NH - NW - NX - NZ - OA - OK - OM - OS - OU - OZ - PD - PG - PK - PR - PS - PX - PZ - QF - QR - QV - RA - RB - RJ - RO - SA - SB - SC - SK - SN - SQ - SU - SV - SW - S7 - TA - TG - TK - TP - T0 - UA - UL - UN - UO - US - UX - VA - VN - VS - VX - WF - WY - XL - XR - ZH - 4M - 9B - 9W
Interestingly, the flight journey on SQ commenced in Jakarta over 24 hours before the departure time of my connecting flight on AA, so SQ was able to check my bag in for the AA flight before I was able to check in as a passenger on AA.