Yes, just like that, only need to be bigger than 0.5 kWh. That small of a battery will last less than 3.5 minutes at Erzberg, you would have to swap about 10 - 20 of them to make it to the finish line without replacing the main battery.
Such a strategy is doable at Erzberg, maybe even not against the rules, but it would be a negative advertisement for Alta, if the word gets our that each rider had his batteries replaced 15 times. The joke of the day will be that Alta might as well make the bike corded, rather than cordless, so that it could race around a power outlet forever.
I agree that a bigger battery is the best answer. Most of my dirt rides are in the 40-60 mile range making the current Alta battery, barely acceptable to not even close. If I could have a 8.7Kw pack (50% bigger) the Alta would cover 95% of my rides.
At Erzeberg, Alta didn't need much more battery, they made it past checkpoint 8 and I think were going to swap batteries at checkpoint 9. A small pack that snapped in would work well to give a little extra safety margin. Even if the extra pack was only 0.5kw total, if it was changed every checkpoint that would have been an extra 4Kw, or a 69% increase. It also could be designed to just snap in for a race bike with minimal circuitry. I am thinking something like snapping in a new cell phone battery. Just slip in and go so seconds to swap out. Snap it right on the side of the existing pack. As
@Philip pointed out, not sure if the rules would allow it, but for a race bike it could be designed to be a fast boost charge option. The batteries will take the surge in a race situation. I wouldn't want to do it to my battery, but for a one time race bike, the pack only needs to last the race, not 1000 cycles. I have seen video of connecting an nearly empty Tesla pack directly to a full Tesla pack with minimal heating or apparent damage, so I don't think there is a problem technically.
As far as the "joke of the day", I don't think so. In my opinion it would demonstrate the flexibility of the platform and what is worse, snapping in quick charge throughout the race (if allowed in the rules) or not finishing because of a dead battery? In my opinion, having a the ability to give a quick boost and finish the race, is far superior to not finishing.