If HP had gone with webOS we would all be cheering and the tech press would all be asking if HP were mad to try again with a failed OS.
The reality in my opinion is that of all the failures in the webOS story, the one thing that didn't fail WAS the OS!
The failures were (skip this if you've seen it before - oh, of course you have!) Hardware, Finances, Marketing, Management.
If HP had gone with one of the other new or minority OS's, it would either create a flurry of interest or another 'are they mad?' response - and it would also be game over for webOS.
Had they chosen windows, again, game over.
Android is an open source, Linux based system. (open)webOS is an open source, Linux based system. This is why porting to Nexus devices is easiest. If HP can produce a 'nexus-like' phone, it should be possible to boot webOS onto it - in fact, HP should actively ensure it is possible.
By choosing Android as an option, HP are choosing the easiest route to market with the biggest OS player, but it also doesn't necessarily prevent other options - even on the same handset. Dual boot if there's enough memory or a basic OS downloader in firmware or a program like webOS doctor that will load the OS of your choice via the USB cable.
The best hope for HP is to create a solid piece of hardware - the Pre3 is not a bad example. They could still end up as an also-ran in a crowded and competitive market, but an OS choice would be one differentiating factor. Some buyers will spend the first week or so trying the options. Most will choose Android, some might choose webOS and might even like it enough to ask for new apps or even develop them... webOS might start to increase it's foot print. Other manufacturers might try to offer an OS choice. webOS might actually become popular because it would be easy to switch if it doesn't work out for you.
It's hard to predict what HP will come up with as a compelling offer to the mobile market, but openess & choice on the device should not be too difficult. If they make their own 'Nexus', then putting webOS on it should be trivial.