Wire can work, but the risk is that the flux gets on it and it gets brazed to the frame itself. So as long as there is clearance it's an option, but if it gets in the puddle you have a mess. The best options are:
1) the weight of the object; or you use a piece of coat hanger balanced over it like a tightrope walker's balance bar to hold it down. Or use a heavy piece of steel that rests one foot on your BO one foot on something next to it so the weight goes straight down through, etc. Just get creative. When you braze you want a liquid quiet flame, and it doesn't have much pressure and won't probably nock a little chip balanced on a tube off.
2) Not common in the bike field, but they sell silver in ribbon form, check out Brownells. This stuff can be placed between a BO and the tube, and the parts can be clamped, etc... because the silver is already in the joint along with the flux. Give it some heat and it all melts together. It's for mounting sights on guns. Some people do a similar thing by fluxing a small puddle of silver on the tube, fluxing say your pump fitting, hitting both with heat and pressing them together. That kind of deal is nice with pump BOs because they are unbalanced (and unnecessary, but that is just me).