From Apiguard's FAQs
7. Q: Can I feed my colonies whilst using Apiguard?
A: Yes and No. The recommendation is not to apply Apiguard whilst feeding simultaneously in case the bees spend all their time taking the feed and not bothering to clean out the Apiguard gel. This is not a high risk and will vary between different colonies so if you have to feed and treat at the same time, try it in a few colonies first and see how the bees react.
Last year because of lack of time we had Apiguard in an eke, under a coverboard then a feeder in a super.
They filled the eke with syrup-filled brace comb (avoiding the Apiguard tray).
We left the eke on over winter but it was a real pain sorting the mess out in spring.
When our stock of Apiguard is used up maybe we'll try strips instead which don't need an eke.