Is there any possibility to launch ARPWatch on RB/750G hardware using RouterOS, DD-WRT or Open-WRT?
It seems that answer is “NO”:
Any ideas? 
You are not able to launch custom application at MikroTik RotuerOS, perhaps it could be done by other OS, which could be installed by MetaROUTER.
MikroTik RouterOS provides you with /ip arp table, where you can see all ARP pairings, you can change ARP mode and modify the operation of the ARP table.
However, I’m not sure it is efficient enough to get e-mail/notification about every change at /ip arp table.
No, it doesn’t works for me: http://forum.mikrotik.com/t/rb750-g-becomes-inaccessible-after-loading-openwrt/38424/1
No, it doesn’t works for me: http://forum.mikrotik.com/t/mikrotik-arp-default-timeout/25316/1
Sending notifications via syslog seems an ideal solution.
What does not work for /ip arp?
How could arp entry timeout modification help you?
Just an FYI to MT. Adding a native arpwatch daemon inside ROS would help a lot with people using these for PCI/HIPPA compliance…
IE: Add a feature that makes it more attractive to a larger crowd = Sell more… 
We cannot detect fast flip-flops happened between “/ip arp” pollings.
ARPWatch detects these situations “on the fly”.
When timeout is too small, polling must be repeated too often.
For example, at now, when we use “/tool ip-scan”,
Mikrotik forgots first found ARP records early than ip-scan is finished.