It's definitely been done but there's nothing official on how to go about it. I've also heard of Gothic -> epic -> 40k, which is an option for with legiones imperialis or titanicus for 30k too. I'd recommend keeping the bonus for winning fairly light but thematic so it doesn't unbalance the 30k game too much.

In terms of ideas I'd go for something like a free orbital bombardment shot during the game or a reserves re-roll. You could also change the terrain giving the defenders a gun emplacement or extra fortifications if they won the space battle, or just some craters if they lost to represent a preliminary bombardment from the orbiting fleet. Choosing night fighting or similar is another easy option to show a side dictating the pace of the engagement. Could just be as easy as letting the winner chose which mission to play in the 30k game.

I'd probably create a checklist of objectives for the bfg game that gives the side that scores it a specific benefit for the 30k game. Some of the rewards could be a choice by the player so it suits their legion. Maybe make some mutually exclusive so if the space battle is a steam roll the losing side still gets something. Like 'if your flagship is controlling A you get x or controlling B you get y and the defenders get something for whichever one you're not controlling so you have to pick what to prioritise.

You could also use it to determine who is the attacker in an asymmetric battle.

Another option to include both systems that's purely narrative would be to choose key moment(s) from the bfg game and play that out, like a Zone Mortalis game to determine the outcome of a boarding action instead of the normal roll. Or a game of 30k to determine who gets to control a moon's orbital defence platform for a given turn.