Actually RAID10 doesn't exist.. But in reality RAID1+0 is usually refered to as RAID10..
I love RAID.. I've seen some really weird configs... RAID10 spread across multiple controllers.. A nightmare to configure.. Mostly done for performance reasons in heavy duty I/O applications..

The issue would be with the data drives?............you could hotswap the mirrored drives, but you would have to rebuild the striped part of the array?
Sort of yes. You first create 1 stripeset then mirror that stripeset.. If one drive fails you can rebuild the faulty stripeset using it's mirror.. Heck.. you can have 2 faulty drives.. As long as they're in the same stripeset..

[RAID Contr1]-----[Disk1]----[Disk3]
[RAID Contr2]-----[Disk2]----[Disk4]

Disk1+2 in RAID0 and Disk3+4 are the striped mirror of Disk1+2..

Both disk 1 and 2 can die without problems or dataloss. Same with 3 and 4.. But if 1 and 4 dies.. it's restoring your backup time!

When you say "6 drives" I am guessing that you are talking the data drive side of the array?

Like mirrored = 2 x 300 + 2 x 300 = 600 total, and the two striped drives are 2 x 300 = 600?
2x300 in RAID1 for OS.. 4x300 in RAID10 (1+0) for data.. makes 6 in total..

Very interesting!............I have never met anyone who would pay for that
It is expensive indeed.. OTOH drives aren't that expensive anymore.. And some applications can really benefit from the increased I/O throughput (sticking in a bigger cpu and/or more ram is useless in some cases).. while still maintaining fault tolerance...