Mike,

 > PPP disconnect and then poll() returning POLLHUP:
>

Sounds like you confirmed my suspicion.

>
> Any simple ways to fix this, and are all Huawei USB modems subject to

No simple ways.  I have many Huawei modems and only a few exhibit this 
behavior.  Perhaps try to upgrade the firmware?

> this? This was for prototyping on an embedded system, can anyone give
> recommendations for good modems that are known to work well with
> ofono?

We've had good luck with Option USB sticks and a several from Sony Ericsson.

Regards,
-Denis