The generic URI syntax actually specifies that any path segment within a URI can be divided up into parts by semicolons (or commas), and suggests using key=value as parameters within that segment.

http://tools.ietf.org/html/rfc3986#section-3.3

   For
   example, the semicolon (";") and equals ("=") reserved characters are
   often used to delimit parameters and parameter values applicable to
   that segment.  The comma (",") reserved character is often used for
   similar purposes.  For example, one URI producer might use a segment
   such as "name;v=1.1" to indicate a reference to version 1.1 of
   "name", whereas another might use a segment such as "name,1.1" to
   indicate the same

There’s no thought given as far as I can see to the relative URL concepts this matrix proposal covers, but the idea of adding key=value data in this format is supported by the RFC, and indeed in a way that supports further adding sub resources below the current one.

Which, by the way, this matrix relative URL proposal seems to ignore - how is ./subpage resolved relative to /map;x=250;y=582 ?