I have 2 doing that, on R410/R710,  it boots to SSD, win2012R2 in fact, using the useless DVD drive port. works like greased lightening. allows me to use all drive slots for huge RAID 6   ESXi is even more easy, they excel in this way, booting from odd things, even USB ports.   why not just try?  rack out all drives, and boot that slot to freedos, see it works then boot to any OS and see it works  I tested min all possible ways (OS) to see what can do, called lab testing.  takes no time at all to do tests. just do it.
Hi, PowerEdge T320 Can I use ODD2/TBU sata port to connect 120GB SSD for booting ESXi, along with power from backplane.   Thank you Abhay  
Poweredge T320 ODD2/TBU