Okay, so good news all the way around-- BOTH solutions actually worked.
If I re-ran my above process as-is but ONLY added the "-NoSignatureCheck" to the end of the export command, it worked just fine.
If I re-ran my above process as-is but removed the HPVSA mode (Remove-ESXSoftwarePackage -ImageProfile HP-Cisco-ESXi-5.0.1-standard -SoftwarePackage scsi-hpvsa), that ALSO worked- and I didn't need the "-NoSignatureCheck".
(Oh, and I didn't need CommunitySupported either)
Since we may have a bunch of new stand-alone hosts (ROBOs) that will have local storage in the near future, I decided to leave the HPVSA in and just used the -NoSignatureCheck.
Below are my complete working steps to get the the Cisco Nexus VIBs injected into the HP Customized Installer. Thanks all for the help!
1) Download HP's Customized Image Profile ZIP from VMware's Website
2) Open PowerCLI
3) Add-EsxSoftwareDepot .\VMware-ESXi-5.0.0-Update1-623860-HP-5.20.43.zip
4) New-EsxImageProfile -CloneProfile HP-ESXi-5.0.1-standard -Name HP-Cisco-ESXi-5.0.1-standard
5) Add-EsxSoftwareDepot -DepotURL https://hostupdate.vmware.com/software/VUM/PRODUCTION/csco-main/csco-depot-index.xml
6) Add-EsxSoftwarePackage -imageprofile HP-Cisco-ESXi-5.0.1-standard cisco-vem-v140-esx
7) Compare-EsxImageProfile -ComparisonProfile HP-Cisco-ESXi-5.0.1-standard -referenceprofile HP-ESXi-5.0.1-standard
8) Export-EsxImageProfile -ImageProfile HP-Cisco-ESXi-5.0.1-standard -exporttobundle -filepath .\ESXi-5.0.1-HP-Cisco-623860.zip -NoSignatureCheck
9) Export-EsxImageProfile -ImageProfile HP-Cisco-ESXi-5.0.1-standard -exporttoiso -filepath .\ESXi-5.0.1-HP-Cisco-623860.iso -NoSignatureCheck