Can you marinade with tallow?
Theoretically yes, practically, it will serve no purpose.

Marinades serve the purpose of drawing flavors out of the aromatics and herbs and distributing them to the food being marinated. To do so, they need to be in a form that can absorb and re-distribute.

There are so many opinions on marinating...you know, everyone has one.... but the other marinade components are often acid, stock or wine. The herb and spice flavors are more fat soluble than water soluble, so acid is added for dubious tenderizing benefits (for short time periods) 

The pro way to prep meat is to have it come to room temperature before preparing. In this case, if tallow is liquid at room temperature (hot room, I think), you might get away with it, but would have little time before the emat needs to be prepared.

Even I will not advocate for meat left out at warm room temperature for an extended period. 

Basically, once the tallow can do the job of a fat in a marinade, it will not have enough time to do anything useful.

PS. You could sautee some aromatics in tallow, then deglaze and make a mushy pasty mixture that you can slather on something, maybe in a plastic bag. If you turn and massage it every couple hours something good will happen, but in a refrigerator it will get quite hard and difficult to work with.