Back in the ‘70s, we got to see multiple incarnations of Doctor Who team up for The Three Doctors, which eventually results in them coming up with a plan: “We’re all agreed, then; risky, but it could work.” “We’d have to strip down the forcefield.” “But that’d mean leaving the Tardis defenseless!” “Yes, I know; but I think we’ll have to risk it.” “It’s worth a try. Come on!”
We then see other folks try to get in on the act: “Would you mind telling us laymen what you scientists are getting so excited about?” “We think we’ve found a way of dealing with Omega.” “Mind telling me how?” “Later, old chap. If there’s still time.” “Oh, splendid.” “How do we know that he’ll take the generator? What if he should refuse?”
…but then The Second Doctor finds a musical instrument, prompting The Third Doctor to hatch a new plan: “Don’t you understand? It’s exactly what we want! Far better than the forcefield generator!”
Here’s what makes it far better: if it comes into contact with Omega, or with stuff near Omega, annihilation of the matter-in-contact-with-antimatter variety will ensue.
And so they go with that instrument-based plan, and, sure enough: antimatter, matter; annihilation, hooray. But: what was Plan A? On a Watsonian read, there must’ve been one, which would’ve been far worse than what they went with; but, unless I missed it, nobody bothers to specify the details. And the Doylist answer could be that the writers never bothered to come up with details; but I’m curious as to whether that’s so.
Since the forcefield generator is what shielded the flute from being converted into antimatter along with the rest of the TARDIS and it’s occupants, I speculate that they were going to try to somehow use the generator to invert Omega, leading to his annihilation with his surroundings. The flute saved them the trouble.
That sounds great, but the “somehow” is pretty much doing all the work, there.
As far as I can tell, the story so far is that (a) Omega sought to convert the TARDIS and its occupants into antimatter when bringing them into his antimatter universe, and (b) as you say, the recorder that was in a forcefield at the time was shielded from Omega’s conversion attempt…
…but on that read of it, the forcefield generator doesn’t actually invert anything; it can shield an item from becoming inverted, it can keep things as they already are, but: so what? To switch sci-fi contexts for a moment, it’s like putting in earplugs to block out hypnotic commands, or brandishing a cross to keep vampires at bay: that wouldn’t mean thereby gaining hypnotic powers or vampire powers or whatever; the ability to stop something from happening doesn’t imply the ability to make it happen.