You can raise the window associated with the dialog`s shell. This is one of the few occasions on which it is appropriate to use Xlib calls to manipulate widgets; Xt has a bit of a hole about raise/lower requests, including for top-level windows.

Note that the window manager has ultimate control over the request, which may well be denied. In this case, it sounds like you`re not using mwm, which places dialogs above their parents, in any case.