Is there any way for moderators or voters to affect the answer selection process if OP asks a question and doesn't ever return to the site to select an answer?
The "answer selection process" has two parts.
One part is only ever controlled by the asked of the original question when they select which answer they perceive as helping the most.
The other part is voting and the resulting scores and sorting. This always in the community's hands. Moderators get one vote each. Users get one vote each. The original poster gets one vote. The resulting vote tally is a crowed sourced ranking of how useful the community perceived an answer to be.
Don't sweat it if the OP doesn't ever do anything about the first one. The second one is really how SE sites work and the most useful indicator to future readers anyway. Do your duty and vote frequently and you will more than make up for drive-by posts not using the tools they could have.
Unfortunately no, to my knowledge. The good news is that voting will help distinguish answers' usefulness and thus serve to help distinguish the best answers. Even though no answer may be accepted, the answer(s) with the most votes are ones the community clearly finds the most useful, and users can even award bounties on others' questions to attract more/better answers or to reward existing ones.