I'd say it's probably simplest to just do "Resolved Document" -> "For Styling" from the Edit menu. The process to build it uses the composition pipeline with a fair number of steps along the way, so I don't think there'd be much bang for the buck in trying to recreate the function outside of Editor. You might be able to find a copy in .aptcache, but that would only be as up-to-date as the last publishing operation.
I've noticed on at least one project, where I was using this function fairly often, that Editor (6.0, anyway) seems to leave a copy of the generated RDS on disk next to the DITA map, gradually accumulating quite a collection of sequentially-numbered versions.
Does anybody know, is there a means of testing if a COM handle is still valid using ACL? That is, using a COM handle (integer value) that was originally returned from a call to com_attach can we evaluate if the resource is available based on that handle?
If you need to do it from ACL, there are functions in $ARBORTEXT/packages/tools/comp_createprds.acl that can help with that. They are undocumented, so use at your own risk (they may change in future releases), but they can make life easier if you need to build a resolved document for styling as part of some ACL code.
If you don't mind the resolved document popping up in a new editor Window when you generate it, you can just use the "EditDitaRDS" alias, which is the command linked to the File->Edit Resolved Document->For Styling.