I have several VM's running on a cluster of ESX3 servers. I know there are a few options for network mounted USB devices. In my situation all of the VM's are off of different virtual switches, or the ones that matter are. For security reasons I can't have any device bridging the networks for USB device hosting. I also don't want to get up and go plug in the networkcable to the correct switch each time we do file transfers. Is there a good solution for this ? Also some times the files are bigger than a floppy will hold, so I don't think the virtual floppyimage will solve my problem.
Here's a few things you can try.
Build yourself an ftp server that is multihomed to each different ip or vlan that corresponds to each secure VM. Use multiple virtual NIC's on the FTP server, one for each server you are transferring files to. Also leave one virtual NIC assigned to some type of address you can get to on a regular basis.
Once you have all the NIC's setup and make sure you don't cause some kind of routing problem, disconnect the NIC's in the VM that you don't want connected, then none of the VMs will be able to talk to it. When you want to transfer files, reconnect one of the NIC's that can talk to the VM you are transferring files with.
Depending on the direction of the file transfer, there are two things you can try.
If you are looking to "push" files from a management workstation to your guest machines, I'd suggest using ISO images. All desktop operating systems have software for converting directory structures into ISO files, which can then be transferred to the ESX server and mounted in the virtual CDROM drive of your guest.
If you need to "pull" files off of the guest machines, the other option is to use a virtual disk. I haven't tested this since ESX 2.0 but, create a new virtual disk in your guest, for personal preference I place it on the second virtual SCSI bus (1:0 etc). Most modern O/S's support the dynamic discovery of disk devices these days, so you don't even have to shutdown the host. It will then operate as a local device in your guest for moving files to and from. You can then either create a new guest in the same subet as your management workstation and move the disk over to it when you need to access the files, or you can FTP it to your management workstation and mount it using the disk tools VMware provides.
Just two thoughts, hope they give you some ideas of your own. Unfortunately if security is as highly valued as your post seems to suggest, you may need to accept the legwork involved in a manual physical task like network patching to preserve that security.