qgis - On the fly transformation From WGS84 to NAD27 with proj4 code

I want to use this proj4 code: +proj=utm +zone=19 +ellps=WGS84 +toclrk66=-3,142,183,0,0,0,0 +units=m +no_defs or this one +proj=utm +zone=19 +ellps=WGS84 +tonad27=-3,142,183,0,0,0,0 +units=m +no_defs

...to make on the fly transformations from WGS84 to NAD27, using the geocentric tanslation parameters recommended for Dominican Republic. I typed this codes using the "New CRS" capacity in QGIS 2.0 (Configuration>Custom CRS), but it doesn't accept the transformation parameters. After typing the codes, and pressing OK, the CRS created looks like this: +proj=utm +zone=19 +ellps=WGS84 +units=m +no_defs

QGIS deletes the geocentric traslation parameters. Maybe I'm violating proj4 codes rules, and that's why I'm asking for help.

I looked up for a solution amongst similar answers on the forum, but couldn't find anything. Sorry if I'm duplicating a question.

Any help would be appreciated.

Jose