I'm proud of Compiere and I'll like it more with DB Indipendence/Opensource-full-choice.
But I'm a bit confused.
The big issue seems to be a bunch of stored procedures/nested transactions.
So, the effort to have a CMP version will be enormous, in order to "disassemble" the stored procedures.
Maybe it will be easier and faster to implement another db with a similar procedural language (firebird? sapdb? pgsql?)
Which way have you chosen?
Is the work actually started?
If you tell us what exactly the real status, willings and needs maybe we'll help someway (paying also with our own developing-time).
PostgreSQL is the only community project in the domain of database. Other projects are some way or another commercial projects or spin-off of commercial projects. Therefore, they do not advance as fast as PostgreSQL, which has a major release every 6 months.
If you would like to consider PostgreSQL, you may want to try our new IDE called pgAdmin3 from http://www.pgadmin.org.
Do not hesitate to contact me directly from our developper page. I will be able to give you more detailed information about PostrgeSQL.
In short: in a clean code, nested transactions are never needed. The only solution I see is to fork Compiere database, port it to PostgreSQL (which can be done quickly in pgAdmin3) and then submit it to Compiere developpers.