UPDATE: options 150 (or 66) are often not needed to use a separate TFTP server, the original siaddr field should be enough if configurable on the DHCP server, see at the end.
RFC 5859 tells this:
This memo documents existing usage for the "TFTP Server Address"
option. The option number currently in use is 150.
There are two commonly accepted methods to discover this server via
DHCP; the "sname" field in the DHCP header [RFC2131] and the "TFTP
Server Name" option (66) [RFC2132]. Both of these sources of
information, however, contain the TFTP server's hostname. That
hostname must then be translated to an IP address. The usual method
to accomplish this would be DNS [RFC1034].
- TFTP Server Address Option Definition
The TFTP Server Address option is a DHCP option [RFC2132]. The
option contains one or more IPv4 addresses that the client MAY use.
The current use of this option is for downloading configuration from
a VoIP server via TFTP; however, the option may be used for purposes
other than contacting a VoIP configuration server.
The format of the option is:
Code Len IPv4 Configuration Server Address(es)
| 150 | n | IPv4 address | ...
The option minimum length (n) is 4.
So I can only tell the answer to your question is: no, you can't use option 66 at all for this, you should support DHCP option 150 or the siaddr field which is the "next-server" configuration pointing to the TFTP server, but this RFC mentions some limited DHCP servers will only return themselves in this field.
UPDATE (to clarify the solution might be simpler than thought):
If providing the correct siaddr (ie different from the DHCP server itself) is supported by the DHCP server (eg: ISC dhcp, or busybox's own udhcpd), then it's of course already supported by the udhcpcd client:
The paramaters for enviromental variables are as follows:
$HOME - The set $HOME env or "/"
siaddr - The bootp next server option
siaddr is what I've been using so far to boot x86 systems over DHCP with a separate TFTP server, for example straight to a Debian network installation. I didn't know about nor need options 66 or 150.