Untested, but it should work in theory:
- Be able to raid (via religion or culture).
- Set up some raiders near the MR.
- Become hostile to the MR (eg. failed imprisonment, attempt to revoke title, attempt to retract vassal).
- Raid the county containing the trade post.
- Sieging down the top holding as a raider should destroy the SR trade post. (This is, arguably, a bug.)
- Win the war as normal.
- Build the trade post immediately.
Alternatively (also untested):
- Become hostile to the MR (eg. failed imprisonment, attempt to revoke title, attempt to retract vassal).
- Siege down every holding in the county, including the SR trade post.
- Sieging down the SR trade post as a non-raider should destroy the SR trade post. (This is, arguably, a bug.)
- Win the war as normal.
- Build the trade post immediately.
(This is from frequent early-game experience of
defending against raiders and/or sieges. I've become quite sensitive about people sieging-down and destroying my lovely fully-upgraded SR trade posts.)