FYI, I was able to successfully use bpjones process to create a custom ISO. In addition, instead of using an 'older' BCM-NetXtremeII-3.0-offline_bundle-1566459.zip Nic driver, looks like Broadcom as released an updated one a few days ago which is the 1.7a.x driver with the only fix to change the driver version to 1.710.x (which will work with VUM). BCM-NetXtremeII-5.0-offline_bundle-1962044.zip
I tested this successfully on a BL460c G6 & BL460c G7 to upgrade from esxi 5.0U3 --> 5.5U1 and I also imported the driver directly into VUM and was able to update a 'march-iso-upgraded' host (gen8) successfully.
Here's the Download and a snippet release notes.
Version 1.710.10.v[50,55].1 (Jun 24, 2014)
====================================
Internal FW 7.10.7
Fixes:
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1. Problem: (VMware PRs 1268606 and 1273517) VIB update failing due to
driver version.
Cause: The driver was using string 7a in the driver version. The VMware
update manager considers this string as lower than the versions
used in previous async/inbox drivers and hence update fails.
Change: Changed string 7a in driver version to 710 to allow update from
previous async/inbox drivers.
Impact: Package update.