Connecting a feeder to main lugs?

[Nutmegger]

Scenario: An MLO subpanel had a main breaker installed, so the main lugs are no longer used but are still accessible (this is a GE Goldmark loadcenter).
Looking to feed a 40A GFCI spa disconnect from that sub. I can connect that feeder to a 40A breaker in the sub, but to save those 2 positions in the panel, can I tap the feeder off of those unused main panel lugs?
The panel is OCPD'd at 60A, and the conductors for the spa disconnect are 4 gauge AL, so that's still within the panel's main breaker ampacity.
Does anyone see an issue there, or would you say "play it safe" and not use those main lugs?

 

[jaggedben]

I don't see an issue. I see this sort of thing frequently, although usually it's feeding a subpanel.

If someone wanted to backfeed the panel with solar it raises issues, but you didn't mention anything like that.