Hi Dave,
Yes you are right in saying that SAP GTS doesn't actually integrate with Project Systems module in ERP. So that means information related to Projects can't be directly passed onto GTS. But may be we can indirectly achieve this License check based on Projects.
GTS Documentation on Maintaining Projects says that after creating Projects within GTS, you can link the Logistics Documents to these Projects using a BADI /SAPSLL/IF_EXTEND_CON_CDOC. Although the purpose is purely for reporting i.e. to see which logistics documents have been created under a Project, however the fact that these Projects are attributes within the license, we can may be perform license determination based on these.
Logistics documents in ERP will include Project Number and these can then be mapped to GTS Projects via BADI, Projects can be added to Licenses and then License Determination can be performed. This is just a thought at the moment but will have to be tested for feasibility.
Thanks,
Shobhit.