I run EggCatcher and MonsterBox on my server and you can do exactly what you are describing. In no way do I consider it cheating. In fact, if you have the skill and desire to farm larger than average quantities of wither skulls and thereby nether stars, I encourage you to do so. Overall, the game balancing implications of such modifications have minimal effect on PVP, particularly if no additional economy plugins are present which would allow you to exchange items into currency. If economy plugins are present a value adjustment to wither skulls or nether stars might be taken into consideration.
It is very polite of you to have considered the ethics of the situation in the first place, so you seem like a fair and valuable member of that community. That said, it is of course at the discretion of individual owners/servers and perhaps even defined by the technical limitations of their plugins. (ie, some mob egg plugins have no way of regulating that behavior)