As of this writing, the hardware sequencing and looping market is sorely lacking implementations of these two very useful MIDI Messages defined in the MIDI Spec.
The days of hardware Loopers or Sequencers capable of only running in 4/4 should be long past, but despite having gear on the market which can support odd Time Signatures, the Time Signature and Bar Marker messages are often, strangely, absent.
Time Signature
I think this isn't as well know as it should be because of the lack of implementations, but MIDI does have within its specification the ability for gear to transmit Time Signature changes. Just like they have the ability to transmit Start, Stop or Continue. It is defined under System Exclusive - Notation Information.
The spec even calls for Time Signature changes to either be acted upon immediately or quantized to the next Bar (this is where the Bar Marker message comes in a bit further down).
The reason users would want this seems fairly obvious: If you have gear that is synced together via MIDI, it can be extremely important that their bars and/or loops remain tightly coupled with each other. Instead, too often you must go through each piece of gear one at a time and set their Time Signatures manually, not to mention how difficult it can be to manage dynamic Time Signature changes during a composition on several pieces of gear.
If the gear already recognizes Time Signatures internally, then sending or receiving MIDI Time Signature messages is pretty trivial and shouldn't be left out of a MIDI Implementation.
Bar Marker
This one is even more rare than Time Signature. This one goes hand-in-hand with MIDI Clock for devices that support recognition of Bars.
Bar Markers can be useful in several ways. The main intent was for the controlling piece of gear to transmit a message at the top of every Bar to indicate to other gear that the next Clock pulse will be the Bar Line. This message would include a number so that gear could also recognize a location in the composition, useful if there is a command that instructs gear to snap to that location or is waiting to execute at a specific Bar.
Bar numbers are optional though. Bar Markers can still be sent and received by devices that recognize Bars, but don't use them for long running sequencers, such as Loopers. A Bar Marker without a number is still a handy thing to drive quantized features and commands. For instance, a device receiving Bar Marker messages could have a command cued up in it, waiting to execute.
Bar Markers are also tied directly to Time Signature since the MIDI Spec calls for the ability of devices to change their Time Signature when the next Bar Marker is received.
Do the Spec
This is my appeal to the designers of MIDI capable gear such as Controllers, Clocks, Loopers or Sequencers. Look at these two standard messages. If you don't really know what they'd be used for, don't worry about it and put them in anyway. Some enterprising musician or other developer out there will make use of them and thank you for it. The more gear making use of these simple ideas, the more common the knowledge will become.