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.