Hi IanT,
I am only thinking out a loud here.
Option1
I would explore possibly the avenue of setting up on a print server a shared printer with double sided set as its default so instead of encoding within your application. Then direct the application to the new shared printer set with default double sided printing?
Option 2
I have dealt with HP Escape sequences in printing from DB2 applications. The only problem with this solution in this case is that if the HP printer changes a whole host of new escape sequences are required, not sure what your situation is.
If you contact HP they normally do not give the escape sequence details on normal purchase but will if you ring them direct, I got mine for free but maybe that has changed as it was a long time ago.
I think in this case exploring option 1 is better as it does not restrict you to one type of printer(HP). I thhink it is more feasible as the same printer can be shared over a network with different names but different settings as thier defaults in your case double sided copying.
Good Luck with it.
Robert88