Sorry I forgot some stuff,
1) This is a common asked question and doing a search of the forums might give you some good info for your game.
1a) I do not remember any official rule in RM1-2 or RMSS/FRP for this issue, besides the suggestion of using acrobatics for general maneuvers and combat Flying to limit other skills while flying.
1b) A number of times people were going to write articles and submit them to theGuildCompanion.com but I do not know if that was done.
1c) I do remember one person saying they were going to use some rules from D&D 3.XX in their game but I do not remember exactly what they were.
General Notes:
1) Concentration: I do not remember right now but it might be a good idea to do some serious playtesting and or add flying actions to a % activity chart to provide penalties to actions.
2) Moving Maneuvers: In general, like you would if a caster was running, jumping, etc you should adjust the casting maneuver accordingly to actions taken.
Thus: Horizontal layout, head rolling, arms flailing should be a penalty to most actions in RM (except maybe acting in a comedy to get laughs).
3) IMHO this topic could have 2-30 pages or material written to try cover all the possibilities of this unique area of combat, just like underwater combat, blind combat, space combat, any some I probably forgot.
Now that I think of it maybe look at SM:P and how they do space/weightlessness actions/combat for ideas. I do not remember them well enough right now to comment.
MDC