The easiest way to get a fieldcode correct is to create one in a piece of text, then use
(vla-fieldcode (vlax-ename->vla-object (car (entsel))))
to extract the code exactly how it would be formatted.
FWIW, I'd step through the layout data, would be a lot faster...
(vlax-for layout (vla-get-layouts (vla-get-activedocument (vlax-get-acad-object)))
(if (not (eq (vla-get-name layout) "Model"))
(vla-put-textheight
(vla-addmtext
(vla-get-block layout)
(vlax-3d-point '(0. 0. 0.))
0.
"%<\\AcSm.16.2 SheetSet.ProjectName \\f \"%tc1\">%"
)
0.1
)
)
)