I believe having a "reserve fleet" would penalize careful positioning/use of sensor mechanics specifically geared toward getting the station on its own. A battlestation is doable with a mid-game fleet (on its own) but once you start throwing fleets in there, it becomes much more difficult. Star Fortresses can handle end-game fleets fairly well by themselves and they're rarely alone.
All that said, if reserve fleets were permanently attached, I think opposing mechanics could also be brought to bear. For example, a "sneak attack" would be possible if your sensor profile was some % of the stations (i.e. Go Dark or attacking from a nebula, etc.) and this would negate the reserve fleet. Likewise, a boarding action could sabotage the station's reserve fleet prior to the engagement.
An alternate idea, which may or may not be doable, would have special Station Patrols that are clearly marked. These always stay within the station's "ally" radius so attacking any of these fleets (at its default range) also engages the station. However(!), they can be baited outside of the station's ally radius, and if you gank them, it has some deleterious effect on the station itself. For example, on a Tier 1 Station, if you take out the Station Patrol, the station suffers a 15% received damage debuff (i.e. it takes 15% more damage). Higher tier stations have one more fleet per level, but each fleet destroyed further debuffs the station. These Station Patrols are not quick to be replaced (30 days?) like standard patrols.