In a comment on this question it was pointed out that the combination "をだけ" appears a lot in a Google search, despite my thought that it was ungrammatical. On closer inspection it seems that many of these hits are of the form をだけを. Here's an example:
It's no good if you're only looking out for your weight (not sure about my translation)
It seems to occur far too often for this to be a typo.
What's going on here?