Joist hangers to use for rough cut 2x8 (2 3/4" x 8 3/4")?
I'm leaving my original answer below in case anyone is trying to hang a 1.75" board, but the question does clearly state 2.75", so, here is another option.

They do make joist hangers that are meant to hang doubled-up joists.  These will be wide enough to handle 3 1/8" (hanger dimensions are here and have 1/8" of play for the doubled up joists that should be 3").

The extra 3/8" of width will be divided to 3/16" on either side, which shouldn't be a major concern.  You could cut some 1/4-3/8" shim stock if you're worried about the slop.  Strong-Tie also makes "rough sawn" hangers, but they measure 2" and 4" respectively (single and double hangers), so I think the standard double hanger is best here.




ORIGINAL ANSWER

The only thing that comes to mind immediately are decorative joist hangers used for decks and pergolas such as these.



These are made for decorative cedar that doesn't conform to the 1.5" standard that all framing lumber uses.  If you look at the specifications for these particular hangers, they will accept lumber that is sized from 1.5" to 1.88".  Your 1.75" fits nicely in that range.  A standard joist hanger will only accept 1.5", as you have figured out.

Now, that one brand is just a random example that I have never used but your local home store and other online retailers should have similar products.  You do need to get more detailed specs on the hanger you choose to determine its structural ability.  These are not purely decorative - they do need to hold together pergolas and the like, but I am not sure if they are as strong as the typical galvanized joist hangers that are normally used.