Forgive me for not reading all the posts (in case anyone else has made this suggestion), but if this were me, I would make a baseplate for the T-posts (maybe 18" square), and on the bottom of the plate would be (5) 6" to 8" spikes that would go into the ground (rebar would work well), one in the center and one at each corner.
As you're using these to hold banners, you could even go one step more simple and have a baseplate that has only (2) spikes into the ground; one under the post and another facing inward (towards and under the banner) about 18" or so. Doing it this way, your baseplate could be much narrower and lighter, although there would be little front/back support against wind hitting the banners.
