All you need is a long m5 bolt with two nuts, and perhaps a few matching washers to make a stem spreader.
The bolt goes through the stem with the nuts and washers in the middle... back the nuts away from each other and this will spread the stem just a little and hold it that way while you are installing the bars.
My friend and I figured this out the other day when we were extracting a Cinelli bar from a Nitto stem.
I am having trouble envisioning this procedure. You wouldn't happen to have a picture of the tool in use, by any chance. I would like to find something that works better than my home made pry bar, which actually works but is a bit awkward to use.