how to use a disk with high reallocated sector count?

You don't.

You go buy another disk to replace it unless you just really like losing data.

I'd like to thank you for the advice and share some of the details that I've got from experiments.

In short, there is no easy way to get the list of reallocated sectors and even statistical methods of mapping the disk are heavily encumbered by the need to play against the logic of the firmware.

To test the drive I ran badblocks -wv with the default blocksize and monitored the reallocated sector count in the process. I made several observations.

1. I observed that there was a sharp rise in the number of reallocated sectors when writing to the beginning of the disk. Then from the first 10G to 700G there was no change. This can be explained by the fact that certain RAID houskeeping data was stored at the beginning of the disk, therefore the wear in the small addresses area was higher than in the rest of the disk.

2. Then after a single error the disk turned itself into a blocked mode. That is every ATA command, even IDENTIFY DRIVE returned ABRT. Even though the value of reallocated sectors was still positive. To explain this behaviour as David Schwartz suggested, I assumed that reserved sectors are somehow distributed over the address space of the drive. This means that the drive might have reserved sectors, yet a part of it may run out of sectors to remap. In this situation the firmware just blocks the drive.

3. The drive returns out of the blocked mode only after powercycling the drive. When the old drives let the software keep track of bad blocks and avoid using them, modern drives do not give this opportunity. When the firmware thinks it cannot cope with the errors, it makes the drive unusable.

4. By running the value of reallocated sectors down to 02 I conclude that there are 2048 reserved sectors on this drive.

5. So-called low level formatting, or writing zeros to every accessible sector of the drive to reallocate the sectors from less reliable parts of the disk would not work because when the drive runs out of reserved sectors it changes the way it handles errors in a way that makes it much less convenient to use than a traditional drive that does not do any predictive failure analysis and simply reports an error.

If you have business data that is worth less than the cost of the drive then use them for that, if not then throw them away or give them to people from the department who understand the risks. Contact the manufacturer and see if they offer recycling.

If the drive is still under warranty, you can return it to the manufacturer via their RMA process for a free replacement, after sanitizing it first. (Secure Erase will wipe the entire drive, including reallocated or otherwise inaccessible sectors.) (I'm quite surprised nobody suggested this.) Otherwise, you do what @SpacemanSpiff said and buy a new drive.