Thanks for thinking about it. Yes, this limitation is confirmed (in fact, voluntarily conveyed) by Marketo's professional services team. Landing pages are relatively flexible but not branding domains. You just get one.
I'm glad you agree that exposing the parent brand is not good for your image -- in our case there could even be legal complications if we linked to the parent's domain. Actually we use one of the sub-brands now. This is okay for some of its related brands (kind of like Foot Locker and Lady Foot Locker, no big deal) but for other ones the marketing department is very clear that it's unacceptable. As a result we are now spinning up a new instance, with no shared leads or anything. It's a real pain.
When Marketo creates pURLs it is able to track a click not only to the lead but to the originating e-mail send activity. As far as I know there is no way to replay or simulate what an e-mail specific pURL would be, as it only gets created at send time. If you don't have Marketo create a pURL I don't see how you could retrieve what it would have been and then redirect to it. Not to mention that populating custom pURL fields for every lead -- adding a new such pURL for every lead and unique e-mail -- might be technically feasible but it doesn't matter because the marketing team will never adapt their workflow for this. If you have more suggestions I'm all ears.