Using spotlight on a shared network volume (Time Capsule external USB disk)

I have a Time Capsule with an external USB hard drive connected to it that I use as a shared disk (screenshot Airport Utility: http://screencast.com/t/JvtN2dhui).

I want to search that volume using Spotlight. I've tried everything. I removed the file .metadata neverindex that was preventing the index. I did the sequence:

mdutil -i off /Volumes/Alkimia/
mdutil -E /Volumes/Alkimia/
mdutil -i on /Volumes/Alkimia/

I tried to remove the Spotlight files (don't think there were any, but just in case):

cd /Volumes/Alkimia/
rm -rf .Spotlight*

I added the volume to the Spotlight Privacy locations, closed system prefs, removed the volume from the Spotlight Privacy locations and closed again (as I read this would force Spotlight to reindex again).

I have enabled "guest access" to the disk because I thought maybe root wasn't allowed access on that disk (couldn't imagine, but just in case). See here for a screenshot of Airport Utility: http://screencast.com/t/dt4PqAW55

If I verify the status with mdutil, all looks ok (this is after a sudo su):

sh-3.2# mdutil -s /Volumes/Alkimia/
/Volumes/Alkimia:
Indexing enabled.
sh-3.2#

However, I never noticed Spotlight starting to index it (ideally that would be managed by the Time Capsule I guess, but I don't think it works like that?).

When searching, 0 returns turn up from that volume. I've searched using HoudahSpot to have more flexibility (explicitly include the volume in the search location for example), but to no avail.

I am out of ideas. Is it even possible?