On the face it may not be that crazy an idea since you usually use a hot tub in cold weather when the A/C is off but actually doing it may be troubling.
I don't think you really need "transfer equipment" per se but you will need something that interlocks the circuits.
You might actually be able to do this by interupting the controller for the AC on the low voltage side. (Your AHJ will answer that)
How many amps are we talking about and what size is the branch circuit going to the AC? I bet you a cold one this has a bigger breaker than you can support with the wire if this was sized with the condenser label.
The hot hub will need a conductor sized with 310.16, not 440 (or the HVAC label). Just because he sees a 50a in the panel does not mean there is 6ga wire going out there.
You also find 680 wiring methods are more stringent than 440 and you may need a neutral that is not there.
I really don't know enough about what you have to say yes or know but I know there are several land mines along the way.
I would start with the spa label, then look at 680 and 310.16 to see if the wire/wiring method will work. You may hit the wall right there.