+1 from me too. I happen to know the registry settings, but that is not really a great option. Of course, if it becomes a configurable item, the user should take some responsibility if iClone can't find their content.
Just did some research and this has been requested about a year ago. Peter was going to mention it to the devs. I also found my earlier post with instructions for the registry edits for iClone 5. This was for moving the Template directory for people with a small SSD, but it would be the same for Custom.
Assuming you are on Windows 64-bit:
Open regedit and go to:
HKEY_LOCAL_MACHINE\SOFTWARE\Wow6432Node\Reallusion\iClone\5.0
Among the values are the two main content directories listed: "Custom Data" and "Template Data".
You will want to change "Template Data" to the new location. If there is enough space then it would be best to have Template Data share its path with Custom Data, just to keep things neat.
There are also two shared content directories: "Shared Custom Data" and "Shared Template Data". It's is probably best to leave those where they are.
CAUTION 1: Before you make any changes to the registry, it is best to export the iClone 5.0 node. (Go to File ==> Export.)
CAUTION 2: Most content is loaded in projects and saved, so you will most likely not have problems. Videos are not, which is relevant for certain water effects, but these are mostly saved in the Shared directories. But you may run into issues.
NOTE: If you are on an 32-bit OS the registry path would be:
HKEY_LOCAL_MACHINE\SOFTWARE\Reallusion\iClone\5.0
Hope this will give temporary relief.