Submitted pull request #5 allows formatting controls whose parent is not the uifigure, however, the following fails:

f = uifigure('Position', [680 558 560 420]);
t = uitree(f,'Position',[20 20 520 380]);
tn = uitreenode(t,'Text','Test me');
mlapptools.fontWeight(tn, 'bold');

with the following error:

Error using mlapptools/getWidgetID (line 363)
widgetID query timed out after 5 seconds, UI needs more time to load
Error in mlapptools.getWebElements (line 108)
widgetID = mlapptools.getWidgetID(win, mlapptools.getDataTag(uiElement));
Error in mlapptools.fontWeight (line 65)
[win, widgetID] = mlapptools.getWebElements(uiElement);
Error in Untitled4 (line 6)
mlapptools.fontWeight(tn, 'bold');

(Note that increasing timeout doesn't help)