This is an interesting idea. H1B visas are entirely transferrable, but some employees never find that out (the shady outsourcing companies aren't exactly incentivised to point it out). That said, whenever I've transferred my H1B my future employer has been paying for it - if they didn't, I'd be suspicious about how committed they really are. So, if Triplebyte (a hiring agency) pays it instead of the employer then, sure, go crazy. But it doesn't make a huge difference to me.

That said, I can see opportunity for a hiring agency that is knowledgable about the H1B process and could even refer lawyers to companies. Many new startups have no idea what they're doing, and are turned off from employing people on H1Bs because of it.