I'm not quite clear as to what you're asking. Are you thinking of:
1. Building new packages or customized versions of existing packages that would be distributed?
or
2. Building a site with a customized version of 4.x, letting it run, gather data (bboard posts, ecommerce purchases etc), then port the site - including its data - to the other RDBMS?
Both cases share a basic problem, i.e. the need for two versions of the datamodel and the need to separate out Oracle-specific queries and replace them with PG equivalents. If its new code you're talking about, I'd recommend starting out writing query files and doing your best to split these files according to whether or not the queries are Oracle dependent right from the start. I don't think this would add more than 20% to the task of developing and writing a simple package, and the percentage cost should be even less for a more complex package (because a more complex package requires more advance design work which would be shared by both versions).
Moving an active, running site is another issue, because you have to massage the data as well as port your datamodel and queries.
I only know of one person who's moved an existing ACS Oracle site to OpenACS Postgres, Janine Sisk, and as a matter of fact she did it over this past weekend. I don't think she's 100% done at this point. She spent about all day Sunday on it and got most of the way there, though - that's not so bad. The site's a demo site based on a client site she built last year.
I just checked, she's not done, so I won't pass along the URL.
Anyway, switching data from 4.x Oracle to 4.x Postgers is probably going to be a lot easier than migrating from 3.x Oracle to 4.x Oracle, or Tcl 4.x to Java 5 ...
