Currently in the system you can default a set of shoes for the "running" activity however there is no way to distinguish between road running and trail running for which shoes you want. There are multiple types of 'runs' in the activity app, ie Trail Run, Run, etc however only you can default a pair of shoes for all running. I would say that most people will probably have two different pairs of shoes, one for road and one for trail.
It would be great to be able to default gear/shoes based on specific activities rather than generalized groups