Remotely control a Mac's keyboard from another Mac, without network
The usual way to solve this is using a KVM-over-IP switch (Keyboard-Video-Mouse). This is a general solution that works with almost any type of computer - PC, Mac, whatever.

KVM switches comes in many forms and price ranges from many brands such as for example Raritan, Avocent, Aten, etc. Most have for example 12 or 24 RJ45 connectors where you plugin a normal network cable. You connect an adapter to the far end of the cable. The adapter will provide for example USB and DVI, PS/2 and VGA or similar connectors that you plugin to the computer you want to remotely control. 

Cheaper KVMs also exist that uses USB or proprietary cabling, provides fewer ports, etc. 

You can then from your desktop PC at any location connect to the KVM switch over the network and control keyboard/mouse and see what's on the remote computer's display. Some KVM switches also allow you to create a virtual DVD-drive or floppy-drive over USB that allows you to remotely install operating systems, etc.

You mention co-location centers and they normally use KVM switches for this purpose. However, modern servers usually have built-in management controller using a seperate processor on the mainboard with its own network interface (or piggy-backing on an existing interface). This management controller usually gives you a web interface (or IPMI interface) for remotely controlling the server including video and mouse/keyboard. Usually they also let you power on/off the server and sometimes also providing virtual DVD or floppy drives.

Here's links to different KVM-over-IP products:

Cheaper range:

1-port Lantronix Spider KVM (299.99$ on newegg)

8-port Avocent DSR1021 (389$ on newegg)

1-port Raritan KX II-101 (373$)

32-port Raritan DKX3-232 (3.895$)

16-port Aten CS1716I (661$)