In my last job I tried to handle this in a similar way. The issue we ran into though was that often these managers would not properly evaluate the software. They would get wowed by the sales guys and sign up for huge contracts without, sometimes, even checking with IT or testing other vendors.
Whenever I'm wearing my IT hat at work, a big part of it is being a detective looking for clues that somebody, somewhere, is about to do this. Then I can insert myself into that process. It would be preferable to be included in the beginning, but one must work within the reality of the situation.
