Savepoint and prepared transactions

Hi,
I'm working on a solution that utilizes 2 phase commit protocol (between SQL
Server and PostgreSQL). Normally PostgreSQL statements sequense is:
1. START
2. <inserts, updates, etc.>
3. PREPARE TRANSACTION 'uuid'
4. COMMIT PREPARED 'uuid'

What if on step 2 user application issues statements with SAVEPOINTs, e.g.
2.1. SAVEPOINT svp1
2.2. <inserts, updates, etc.>
2.3. SAVEPOINT svp2
2.4. <inserts, updates, etc.>
2.5. RELEASE SAVEPOINT svp2
2.6. ROLLBACK TO SAVEPOINT svp1
2.7. <inserts, updates, etc.>

Is this allowed and safe to use?

Thanks,
Konstantin