This would be an extension to SPARQL.
1/ Add use of GRAPH inside a CONSTRUCT template see SPARQL Update.
2/ Add conneg for quads to Fuseki.
3/ New QueryExecution operations execConstructQuads() and execConstructDataset()
If asked for triples, and the CONSTRUCT generates quads, the named graph items are dropped - that is, only the default graph is returned. This is for commonality with RIOT.