When I shoot docks, I look for pontoon boats as well ... don't matter if they're moored inside the dock or outside of it ... I shoot the jig under the pontoon. Crappie love shade !!
When I get to the lake early AM .... I cast to wood cover on banks that are shaded. Crappie love shade !!
Do I think your project will work .... .... but, if you anchor it so that it doesn't move from that spot, what would be the difference in it and the pontoon (or a dock) ?? So, yeah ... it probably would work.
But, I'll tell you what I WOULD do .... I'd put it out over some wood cover, while the shade is already over that cover, so that when the Sun starts getting high and the shade from the trees on the bank starts disappearing, your "floating mat" shade will still be covering the spot .... and maybe the fish will hang around it longer, instead of wandering off to the depths of open water.