You're talking about being right on resonance as if it won't help if it's off by 20% of the center frequency. But that's not true. Just like you can use a stumpy monopole whip for 144 Mhz even though it's 6 inches long, and it will help significantly (but behave like an omni capacitive reactance element), so will this not quite right slot antenna mode. There are many ways to excite and feed a slot antenna that do not depend on having the two excitation terminals on each side of the slot. Especially if you're doing it off frequency. Any conductor in the near field is going to couple via induction.