Will a SATA SSD drive work in WWAN (miniPCIe) slot?

Latitude E6x20, E6x30 have WLAN/WWAN slots that are mini PCIe/USB only. They do not support mSATA SSDs.

Latitude E6x40 and 7xx0 have combo mini PCIe/mSATA/USB slots that support mSATA SSDs.

Your IT department may be right. mSATA does not work in any given miniPCI slot. From wikipedia:

The connector is similar in appearance to a PCI Express Mini Card interface,[43] and is electrically compatible; however, the data signals (TX±/RX± SATA, PETn0 PETp0 PERn0 PERp0 PCI Express) need a connection to the SATA host controller instead of the PCI Express host controller.

Put another way, slots that can support mSATA may or may not support miniPCI and vice versa.

The question you're really asking is if the WWAN (full miniPCIe slot) supports mSATA on your specific system, so the Lenovo link in your update isn't relevant - i.e. not all full miniPCIe slots support mSATA.

None of the documentation I've seen from Dell states that the full sized miniPCIe port is also mSATA (I looked for an E6540, since there doesn't seem to have ever been an E6450) . If you really want to convince your IT department, I think you'll have to get a definitive answer from Dell, and show that to them (or maybe you can just try showing them the forum posts about your particular laptop).

There are miniPCIE PATA storage devices but they're pretty rare, and small capacity (e.g. 16 GB or so). The Dell Mini 9 and others used these before mSATA was available.

I know I'm late to the party, but I can confirm that the Dell WWAN slot will accept an mSATA SSD. I just installed one myself and it's working just fine.