I've been trying to get this going on my Linux version of Mystic so far
without success.

After asking around the Synchronet author (DM) kindly pointed me to a copy I could download from Synchronet FTP site.

I've moved it into the Mystic directory and and made it executable

[snip]

./sexyz

Synchronet External X/Y/ZMODEM v3.0 master/32a29523f Copyright Rob Swindell

!No command specified

usage: sexyz <socket> [-opts] <cmd> [file | path | @list]

socket = TCP socket descriptor (leave blank for stdio mode)

opts = -y allow overwriting of existing files when receiving
-k enable X/YMODEM-1K send mode
-c enable XMODEM-CRC receive mode
-g enable X/YMODEM-G receive mode (no error recovery)
-o disable ZMODEM CRC-32 mode (use CRC-16)
-s use segmented ZMODEM (disable streaming)
-2 set maximum ZMODEM block size to 2K
-4 set maximum ZMODEM block size to 4K
-8 set maximum ZMODEM block size to 8K (ZedZap)
-w# set maximum ZMODEM transmit window size (default=0, unlimited)
-m# set maximum receive file size to # bytes (default=0, unlimited)
-l lowercase received filenames
-! to pause after abnormal exit (error)
-telnet to enable Telnet mode (the default except in stdio mode)
-rlogin or -ssh or -raw to disable Telnet mode

cmd = v to display detailed version information
sx to send XMODEM rx to receive XMODEM
sX to send XMODEM-1K rc to receive XMODEM-CRC
sy to send YMODEM ry to receive YMODEM
sY to send YMODEM-1K rg to receive YMODEM-G
sz to send ZMODEM rz to receive ZMODEM

file = filename to send or receive
path = directory to receive files into
list = name of text file with list of filenames to send or receive

[snip]

so far so good,

when I try to run a SEXYZ ZModem download I see this error in the terminal
once I start the download

/bbs/mystic/temp1/xfer.sh 2 :/bbs/mystic/temp1/xfer.sh: sexyz: not found

In the error logs I see

2021.02.09 16:43:35 MYSTIC 001 ERROR: Can't find xfer.log, error 2

I figured if the file was in the mystic root it would be enough for mystic to find it?
--- Mystic BBS v1.12 A46 2020/08/26 (Linux/64)
* Origin: Agency BBS | Dunedin, New Zealand | agency.bbs.nz (21:1/101)

Avon wrote (2021-09-02):

I've been trying to get this going on my Linux version of Mystic so far without success.

Does it make MYSIC?
---
* Origin: . (21:3/102)