As stated in a previous post, you can use the command line (in this case ANT) to build a BPEL netbeans project.
After asking the openESB community, which is very helpful and fast in their answering, it is not possible to get warnings. Only validation errors are presented for this.
Validation itself is configurable with these commands:
Nothing is validated: -Dvalidation=none
Only parts are validated: -Dvalidation=partial
Everything is validated: -Dvalidation=complete
For huge projects it can be better to use the none or partial validation option as the building takes too long otherwise.
References: openESB mailling list regarding this topic