On my T610, by plugging in a PCIe video card, modded so that it fits the x8 PCIe slot, i then get access to the embedded video controller option within BIOS where i can set it to disabled. From that point on, the embedded Matrox video is disabled and all video is handled by the PCIe video card. There are no issues with seeing the BIOS boot sequence via the new PCIe video card emoticon.Smile.title
IIUC, T620 operates in the same way.
About the only issue one should see on the T610/T620 is that you loose some IDRAC features (like boot logging, etc) and that the fans go 75% full speed due to a (presumably not recognised) 75W PCIe card being plugged in.
So, unless Dell has made some changes to T620 System Firmware, then i don't see why the boot sequence can't be seen vis PCIe video or accessing the firmware should be an issue.
Maybe such an issue has more to do with T620 System Boot Mode (BIOS/UEFI) being used in combination with the video card BIOS capability (BIOS when UEFI needed) or UEFI keys or something similar.
Being classic BIOS user, i can say where the issue may lie.