Yes, you can do this. The way around it is to build your own iPXE boot firmware. You will need a linux workstation with git and the usual build tools, make, etc.
Go to http://ipxe.org/download and follow the instructions on downloading iPXE from git.
Build the chainloadable image. Refer to the section titled "Chainloading from an existing PXE ROM".
Place the resulting "undionly.kpxe" file on your TFTP server.
Change the bootfile option for the DHCP scope to the file: undionly.kpxe
Refer to this document on chainloading with PXE and ISC DHCP. Add the following to your DHCP scope options:
if exists user-class and option user-class = "iPXE" { filename "https://<autodeploy server URL>"; } else { filename "undionly.kpxe"; }Where <autodeploy server URL> is equal to the "filename" specified in the tramp file.
To use one TFTP server to work with multiple Auto Deploy instances you will also need to have your ESX hosts on different VLANs and using different DHCP options. But you'll probably want that anyway since having multiple Auto Deploy servers means you're using different vCenter Servers, clusters, etc.