While apparently theoretically possible to mung on the vertices of an annotation object to change the shape --
figure
hF=gcf;
p=hF.Position; p(3)=p(4); hF.Position=p; % make it square aspect ratio
hASq = annotation('rectangle','FaceColor','b','linestyle','none'); % put a square on it, get handle
hFace=hASq.Face; % the hidden quadrilateral object
v=single(zeros(3,4)); % generate set of vertices within the default position
v(1:2,1)=[0.35;0.3];
v(1:2,2)=[0.3;0.35];
v(1:2,3)=[0.4;0.35];
v(1:2,4)=[0.35;0.4]
hFace.VertexData=v; % set the vertices to new data
generates an initial figure that looks like
any time the figure is repainted, the image reverts back to the original square -- there's a lot more behind the scenes than just simply resetting the vertices, unfortunately.
Secondly from a practical standpoint for your purposes, an annotation object and textbox can't be mixed, you would have to draw each separtely and position them such that they appeared to be together.
All in all, methinks this would not be fruitful path to try to follow; not sure there is a good path...
