First off: I understand 64 bit is on best effort, but why? 64bit Linux for x86_64 was introduced about 6 or 7 years ago, nearly all CPU's support 64 bit atm (save a few old/cheap Celerons and Semprons) so I see no point in not supporting it, especially for a server-oriented application. That said, I applaud your efforts regardless of my own personal view

and I think Amahi has a lot to offer.
I've checked /etc/exports, the lines you mentioned are there.
I've also tried the alternate pba backup (SystemrescueCD), and the first time it worked out of the box, but later on I got into 22 errors... there were no permission issues, but the primary disk was full because I only mounted /var/hda/files on the 690GB LV instead of mounting /var/hda there completely grrrr... So I got to learn something new about the hda: it puts all your files in /var/hda/files, but the backups will be done to /var/hda/apps/pba/... which might or might not be on the same disk/partition.
I don't know how much work it is, but it seems better to me to have hda not residing in /var but in a filesystem that tends to be less critical, especially if you need to setup your own disk schemes - like "make sure you have a large partition available and mount it as /hda (or /amahi or whatever). This space will be used for storing your files and backups." /var is not a place I'd choose to have a lot of user files to be placed, when this mountpoint is full you will have no mail, no logging, no database, no printing etc. available anymore.
/home would be an excellent place to have the /hda subdir(s) in if a seperate mountpoint is not possible, if need be you could have a separate hda user account with the same primary gid as apache. It also saves you a lot of hassle in the long run: you don't have to assign a uid:gid in /etc/fstab to a mountpoint anymore, since the /home permissions take care of that. Also for adding a new disks to your system, a lot can be found for /home (add disk, login as root, fdisk & mkfs new disk, mount it as /home and add to fstab yadayada) but not for /var. Someone not so handy on the command line wouldn't be very likely to know that you can do that (or even cooler stuff if you have LVM enabled).