Unfortunately, while the theory makes sense there is little in the way of scientific study to back it up. The first report in the literature was in 1982 (described in the G&M article), and there is a single case study since then that involved guaifenesin use.
Not unexpectedly, that means most of the claims about guaifenesin and pregnancy remain in the realm of anecdote across message boards and in pregnancy books. Also unsurprisingly, many of the comments on the Globe and Mail piece blame the lack of study on Big Pharma conspiracy keeping down a cheap alternative to IVF. The original article is worth a read for a more balanced perspective.