Sure. P(lab leak origin | last bullet) = P(last bullet | lab leak) * P(lab leak) / P(last bullet). There isn't much evidence to constrain each term, and your post doesn't make any attempt to estimate them, but let's make some stuff up. The values change a lot if you take P(lab leak) to be
any lab leak, or specifically a modified RaTG13 from WIV.
Let's go with the more specific scenario. Then P(last bullet | WIV lab leak scenario) becomes P(outbreak | WIV lab leak &c.) because we've conditioned on everything but the outbreak part. I'll guess this is 0.001, which is generous because it's very unlikely for a mere exposure to turn into a pandemic. Exposures to natural potentially pandemic viruses happen every day, most don't become a pandemic. P(WIV lab leak scenario) is difficult. There are a lot of different lab leak scenarios that could have happened instead, but we also don't care that much about trivial differences. Let's be really generous and say P(WIV lab leak scenario) = 0.25 because it's one of I think 4 plausible covid origin sites in Wuhan. Probably it should be lower because it's plausible that Wuhan was only the site of the first superspreader event, not the origin. And let's say P(outbreak) = 0.2 because we seem to get a potential pandemic every 5 years or so. Then P(WIV origin | outbreak) = 0.00125.
There are all sorts of problems with my analysis of course. Even if I haven't made any mistakes, the numbers I conjured up are entirely unconvincing. With only one datum, we have hardly any objective support for our probability estimates, and Bayesian analysis only gives a semblance of objectivity to our prior opinions.
Edit: I have no desire to support either side here, just to point out that probabilistic reasoning doesn't help much here. Whatever happened, happened, regardless of how (un)likely it was.