Once A Fool
I used to award exp bonus's to players who did this in my long time ongoing campaign. I had a list of things that needed work as well as areas that players could create with minimal input from me that I would update as time went on, and it worked quite well (I always had final approval of course). Until... One of my players had gotten a new job assignment and he had a few weeks off of work between his old assignment and the start of his new one. Unbeknownst to me, he went to town. We played every other week and I showed up for one of our sessions and he had pretty much written a books worth of material. It was obvious he has put an insane amount of work into it and it was all really good and it was going to add a lot of really good and valuable material to the campaign, so there was no way I could renege on my ongoing rule. I reluctantly awarded him an insane amount of exp (we had a set formula for how much you got for what work was done). With the exp award he leapt ahead of everyone else in the group. and pretty much stayed ahead of everyone else for the rest of the campaign (several years).
What I took away from it was we added limits to how much could be awarded at any given time as well as a maximum bonus you could receive based on the levels of other players.
I used to use XP as a reward currency, as well, and ended up with the problem illustrated in your post. The problem was, I was getting what I wanted to help make my life easier and it was actually making my life harder! My solution was to change the reward currency to the one described in Lesson 4.
This works better, in part, because the points can be spent on behalf of anyone in the party and, so, help increase the party's survivability--which is great, because, where having a party of mixed-level PCs makes creating combats harder, not having to worry about a TPK as much makes creating combats easier. In a way, I'm rewarding myself when I hand out such a reward.