Actually, there is no requirement that the path be relative to the MT directory. In fact, the central installation model of Movable Type pretty much requires that the plugin directory NOT be under the MT directory, otherwise, it kind of defeats the point.

PluginPath is an ARRAY config directive. This means that it can be specified multiple times allowing you to build up a search path for all of your plugins. Plugin paths are processed in the order that they are specified except for the addons/ directory which Movable Type processes automatically before any others.

One important note: Like all ARRAY config variables, if you specify the directive, you overwrite the default value. Hence, if you want to include MT's plugin directory, you need to add it yourself:

PluginPath        plugins/
PluginPath        /usr/share/movabletype/plugins/
PluginPath        /home/jallen/etc/mt/plugins/