I need to allow longitude output outside -180 to 180 range.
This can be done by:
+proj=merc +over +lon_0=0 +k=1 +x_0=0 +y_0=0 +datum=WGS84 +units=m +no_defs.

See the proj.4 docs on that. It seems +over is not supported by proj4js.

Is there a chance that this feature could be added?
It would be very useful in dealing with certain scientific datasets.