mate, object editing of BIM Server libs is a bit tricky thing because of sharing issues... anyway the process you should follow:
1.open object and modify the subtype template
2.save as... save it to the base office library as you suggested (TW window offers you 'Embedded library' by default so be careful)
3.go to File>Libraries and Objects>Manage BIM Server Libraries
4.select your base office library and 'Refresh' button ... this will effectively upload your new object and let BIM Server know about it... you should get a notification window about how many objects were deleted/updated etc. as a confirmation
5.go to File>Libraries and Objects>Library Manager and select second option under green reload icon 'Reload Libraries and Apply all changes'
now you can create a new object and your custom subtype will be visible.
whole process is a real pain in the neck and I have asked GS to merge (at least) Library Manager and BIM Server Libraries functionality somehow to make it a bit more streamlined.... not sure what rubbish bin this request has ended in though
::rk