I decided to go with the simple approach. We can always fall back to the more complicated solution later if necessary. But I didnt' want to send it straight to net+stable as I would have for normal bugfixes. I'm hoping some of you, in particular those with older SIMCOM modems which always was worked, will pull it from mainline and test.
The patch is currently found here: http://patchwork.ozlabs.org/patch/920501/
I assume it will end up in net-next in the near future. But it won't be in mainline until Linux v4.18, so it would be good to have enough positive test results to request a stable backport before that. That is a hard requirement for OpenWrt in any case, since the next release will be based on v4.14,