Has anyone ever branch transferred as a Junior Grade Officer from a Support MOS to Combat Arms in the National Guard?

I went the other way. I was an Infantry Officer and branch transferred to the Engineers. The state that I was joining asked if I would do that since they had more slots for Engineers. To start, you will need to contact Officer Branch Support at the State Headquarters of the State you are going to serve. They can tell you what is available and get your paperwork going. Generally what will happen is that they will give you at temporary branch transfer tied to completion of your Captains Career Course (I believe that is the correct acronym these days). When I did this, you had a choice of doing the CCC active duty, which often depends on funding, or doing an Active Duty phase, a distance learning course, then a final Active Duty phase to complete it.
As LTC Miller mentioned below, if you are currently assigned to a unit, you will need to start by talking to your CO before you start talking to the state.

You'll probably need to write a memo indicating your reasons for re-branching.
A state only has at most about 4-5 company grade and 2 field grade positions for FC, most states have less, whereas there are 25 company grade and 3 field grade positions in every IN BN. The obvious reason would be career progression opportunity.
And you'll need letter of acceptance from that gaining BN, then G1 OPM transfers you and the gaining BN schedules a new BOLC.

I have not but it is not uncommon. It's more common in the Guard than anywhere else.