When you factor in the cost of the extra pup, switches, pots wiring and labor, you'd be close to the price of buying an HH version of your bass. And you'd have a backup bass to boot.
Or you could put it up for a trade here, and see if you get a bite. Trading it for a HH, maybe plus some cash, will most likely be cheaper than adding a humbucker to your current Bongo.
Not to mention that you'd have to price your home made HH rediculously low to be able to sell it, if you decide you don't need it anymore.
Don't do anything you can't reverse to your bass, is my tip of the day.
I Have seen one,(nearly brought it a few years ago) it was an interesting conversion, with it still having the single H in the same position with just the addition of another another pup and mix pot for the selector switch, it worked well.
Be aware that Music Man does not sell any proprietary parts such as bridges, preamps or pickups.
You're completely on your own with this and would have to use aftermarket pickups and come up with your own wiring, or somehow figure out how to replicate the factory wiring of an HH.
I don't know if the HH preamps are different from the H preamps, but it's likely. The selector switch is wired differently for sure. So at an absolute minimum you're talking about routing the body, shielding it properly, finding and installing a second pickup, and rewiring at least some of the electronics.
You'd wind up with a dual-humbucker bass, but it would NOT be the same as one produced by the factory, if for no other reason than the aftermarket pickup you'd have to use. If you can't duplicate the factory integration of the second pickup with the preamp (and no, the schematics are not available as far as I know), you're DEFINITELY going to have something very different from a production HH.
If modifying instruments is your hobby and you enjoy it, then have at it. It's your bass... go get a router and a soldering iron and knock yourself out. These instruments are tools, not holy relics. Now, I might shed a tear when somebody hacks up a perfectly good SR5 trying to turn it into something else, but that doesn't make it wrong.
Cut up a Bongo, though, and the Collective will gather pitchforks and torches and hunt you down in the middle of the night.
If you're looking for an instrument different from what you've got, you value your time at all, you want a guaranteed top-notch final result and you want it to sound like a genuine dual-humbucker Music Man bass, then the very best, lowest-hassle, safest way to do that is just trade in your existing bass and buy a factory HH.
Unless you're really, really good at this, it's a safe bet that the guys at the factory are going to do a much better job of building a dual-humbucker bass than you will.