If you only have a small number of devices to move and you are using v14.x,  the simplest is to multiple select devices for the whole page of 50 devices from the source device group, and drag the items selected directly to the target group in the device-group tree on the left.  Probably need less than 30 seconds to complete a page with 50 devices, so should be quite efficient if you have 500 or less devices to move.

As you said, you can use relocation rule, but just choose full address range (i.e. 0.0.0.0 to FF.FF.FF.FF) to select all devices in the source group, and there is then no need to add any custom data.  The major drawback of this approach is that relocation of a device will probably only be done when it gets on-line and connected to the MobiControl server, rather than a very short moment after you save and assign the rule.

If the above delay is really a problem, you may also try using an alert rule with trivial (sure-fire)l trigger criterion and the alert action is to relocate the device to your target device group.   A possible sure-fire trigger criterion is "device status" of "available memory" >  0 byte or 1K-byte.  I use something like this a lot in the past.  I'm not sure if it still work in the server version you are using.