This is the solution I came up with using Allen Browne's Concatenate function:
sAssySearch = ConcatRelated("[AssyPN]", "tblBOMr14", "PN =""" & Me.cboSearch.Text & """", "[AssyPN]", "' OR PN = '") & "'"
strFilter = "PN Like " & sSearch & " OR PN = '" & sAssySearch
It took a few stabs to get all the single quotes and double quotes right but it works!