So, here are a few thoughts.
1) There is no automatic method of updating automation folders at this time. This is a known limitation in the product and yes, one of our list of items to fix. There is a KB that discusses how to update automation folders, but the key thing to remember is that you have to essentially remove them and re-install them. The Linux PE and Windows PE "default" items that always show up on the list of preboot configurations are the automation folders. Many delete these thinking "Oh, I don't need them!" Well, yeah, you do. If you add drivers, you then update those, then remove the automation folders, and reinstall them. (not too efficient, but hey, it works.) Alternatively, you can build a SWD job to push a new set of automation folders down to the client, which is what the KB discusses.
Assuming we're dealing with Automation folders, I suspect all we've done is NEVER updated the automation folder directory on the client, no matter how often we re-build the WIM file on the server. It's sounding like that now.
2) There are some drivers that do NOT work in WinPE 2.1. This is a sad but true fact, and is the main reason we have such a big push to get WinPE 3.x supported in the upcoming SP (I hope we make it). There is also a KB that discusses getting a WinPE 3.x WIM file to work in DS 7.1. Basically, you end up replacing the WIM with the new one.
3) As with 6.9, there are times when a driver is "recognized" by the OS but is not correct and doesn't work. In these cases, sometimes moving the driver UP the list within BootWiz will resolve the issue. The bad news is that you have to actually launch BootWiz AND COMPILE in there to get this process to work. <sigh>
4) Personally, I'd test PXE, even if in a limited environment. Why? Because you have a faster time getting to a build of the WIM that you KNOW has been updated. Once you know you have a valid WIM, then send that down to replace the automation folders, and bam - should be good to go.
Anyway, there are some thoughts. If you can't find the KB's, I'll try to revisit this and dig those up as well.