can I snat all the outbound traffic to be one special source port?

Actually from re-reading your first post, it sounds like you're already using this option. Is the command working when there is traffic being sent to just one of the two nodes? Is the source port changing when TMM is trying to send packets to both nodes at the same time?

Edit:

I tested using this rule and saw all packets being sourced from the expected SNAT IP and port:

when CLIENT_ACCEPTED {
   snat 192.168.101.41 666
}

15:25:32.645434 802.1Q vlan4093 P0 192.168.101.41.666 > 192.168.101.45.200: udp 17 (DF)
0x0000   0ffd 0800 4500 002d 73b6 4000 ff11 bc61        ....E..-s.@....a
0x0010   c0a8 6529 c0a8 652d 029a 00c8 0019 2306        ..e)..e-.......
0x0020   686f 7374 2031 2c20 636c 6965 6e74 2031        host.1,.client.1
0x0030   0a00                                           ..
15:25:32.887513 802.1Q vlan4093 P0 192.168.101.41.666 > 192.168.101.45.100: udp 7 (DF)
0x0000   0ffd 0800 4500 0023 73b9 4000 ff11 bc68        ....E..s.@....h
0x0010   c0a8 6529 c0a8 652d 029a 0064 000f ab14        ..e)..e-...d....
0x0020   686f 7374 2032 0a00 0000 0000 0000 0000        host.2..........
0x0030   0000                                           ..
15:25:34.418260 802.1Q vlan4093 P0 192.168.101.41.666 > 192.168.101.45.200: udp 17 (DF)
0x0000   0ffd 0800 4500 002d 73c5 4000 ff11 bc52        ....E..-s.@....R
0x0010   c0a8 6529 c0a8 652d 029a 00c8 0019 2305        ..e)..e-.......
0x0020   686f 7374 2031 2c20 636c 6965 6e74 2032        host.1,.client.2
0x0030   0a00                                           ..
15:25:34.983116 802.1Q vlan4093 P0 192.168.101.41.666 > 192.168.101.45.100: udp 7 (DF)
0x0000   0ffd 0800 4500 0023 73ca 4000 ff11 bc57        ....E..s.@....W
0x0010   c0a8 6529 c0a8 652d 029a 0064 000f ab14        ..e)..e-...d....
0x0020   686f 7374 2032 0a00 0000 0000 0000 0000        host.2..........
0x0030   0000                                           ..

Aaron