Zones up to 11 are part of the series that spans Quebec (center meridian on half degree, zone 11 centered on W82.5). Zones from 12 onwards are part of the series that spans the Prairies (with center meridian on a degree, zone 12 centered on 81.0). And indeed, zone 12 backtracks from zone 11 which is really strange. So in many cases, an explicit center meridian would be needed as explained below.
This page was generated with GPS Visualizer, using these tracks as input, and the following options: width=1100, fullscreen (?), trackOpacity=100, trackWidth=1, waypointStyle=smallTriangles, permanentLabels.
There are two discontinuities in zones, which can be grouped as follows:
To cover problem cases, where the above MTM definitions here don't match your needs, the reference meridian can be explicitly provided when converting to/from MTM. Right now, the false easting cannot be changed, but if there is need, I could add this too. I don't think MTM makes sense on water.
Some specific notes that I have:
Nova Scotia really covers four zones, from 3 to 6, but uses only zones 4 and 5, and extends each a bit into the other zones (at the NE and SW tips of the province) to keep things simpler (my understanding).
Quebec extends zone 3 to its full width, ie. from 57 to 60, and defines a zone 2 (on half degree, ie from 54 to 57 -- so not the same definition as NRCan's zone 2), but there is only a very small part of the province in that zone, and only when using the more extensive border (there is a border dispute here around Labrador).
With GSRUG's 2008 definitions, a very narrow band of Quebec land west of 79.5 appears in zone 12 instead of zone 10 as it should (from Quebec's point of view). This is one place where the "MTM ref meridian" of the tool would be useful (or you can use the QC versions of the tool). I think the border between Quebec and Ontario is just a bit west of 79.5, but GSRUG checks for lon=79.5 exactly (here as well) to switch between zones 10 and 12.
Note also that SCOPQ (Système de coordonnées planes du Québec) uses MTM. Quebec spans zones 3 to 10 in practice.
Ontario is a bit confusing at first. Essentially, two sets of zones (ref meridian on half degree, ref meridian on degree) overlap here, separated by a staircase line separating south-eastern Ontario from north-western. See above image.
Here are the notes I currently have from them.
Alberta also has three-degree zones that extend to four degrees in some cases, while others are limited to 2 degrees. Here MTM is know as 3TM (3 degree traverse Mercator), and they do not use a false easting.
Other provices do not appear to use MTM zones.
Another view of MTM zones from www.posc.org.
Comments/corrections most welcome: (français, English, Deutsch).