As far as I understood, solutions are only valid for transactions that are already known when the block was mined. So it is NOT possible to include more transactions, as this would change the needed solution.
The only thing possible right now is a DoS "attack" by not submitting valid solutions to the network, so it takes longer until a solution is found.
This is useless in solo mode but creates problems in pooled mining currently, as pools rely on you solving difficulty 1 blocks as proof of work, with the hope, that some of these solutions might also solve more difficult blocks. If you withhold the more difficult ones, the pool will still think you do your work perfectly fine, but it will earn less, since your hashrate needs to be deducted from the pools hashrate.
This cannot even be verified via statistical analysis, as the chance to find a solution for the current difficutly is sooooo small, that it won't show up if you just withhold solutions which are also working for difficulty >= current diff.