Charge your Eneloops before storage, and stick them in storage for as long as you want.
I'm running some long-term testing on Eneloop self-discharge characteristics. I'm far from done yet, but I've noticed these trends:
4 year old Eneloops, new out of the package: 72% charged (measured using available energy left in the cells in watt hours). Keep in mind they don't fully charge the cells before packaging them.
1 week of storage, after a full charge: 94% charged. This is also about the same after 1 month. So, most of the initial self-discharge happens very soon after charging.
By 4 months of storage, it's down to about 85% charged. Note that this is energy left, not milliamp hours, and I suspect the mAh would be higher since the voltage is highest on the first part of discharge.
By 6 months, it's still 85%. And it sticks around that for a long time.
So, to conclude, I think you can reasonably expect Eneloops to hold at about 80% - 85% charged for many years, with most of the self-discharge happening within the first few months of storage. When they reach that level, self-discharge basically stops.
That level I measure at about 1.32v to 1.33v. So, if you're designing a system to top up an Eneloop, I'd do it at 1.32v, and it will probably never be reached except when you use the battery. That will keep the battery at about 80% charged.