A couple of thoughts:
1. Maybe the toner darkener could be sprayed onto copper clad after toner has been transferred to the copper clad. That might work as a substitute for Toner Reactive Foil to seal up the cracks and pinholes in the toner on the board. The toner darkener would have to be solvent only. If there is a residue, it might interfere with etching.
2. On the other hand, perhaps laminating Toner Reactive Foil onto the pattern printed on vellum would improve the darkness of the patterns on the vellum, for use as a photomask!