[PROJ] Help with custom CRS proj string

[Daniel Farfán]

I've been trying to solve this problem but was unable to, so I kindly
ask for help.

I have 3 coordinate points in a local grid:
A': 59770.038, 101069.654
B': 59214.93435, 98996.58573
C': 58444.7795, 99231.37298

Corresponding coordinate points in EPSG:32719 CRS:
A: 368362.117, 7062761.917
B: 368318.78, 7060616.253
C: 367514.826, 7060660.088

So, I've determined that to convert  from the local points to the
EPSG:32719 CRS, I have to displace the x axis by 334491.216, the y
axis by 6950332.853, and rotate counterclockwise by 13.8333 degrees.

I have managed to get the following to work:

cct +proj=helmert +x=334491.216 +y=6950332.853 +theta=-49800
When I input the coordinates in the local grid, i get the
corresponding coordinates in EPSG:32719

What I'm struggling with is to use the conversion with cs2cs: i think
I need to define the transformation as a custom CRS, to be able to do
something like:
cs2cs "+proj=helmert +x=334491.216 +y=6950332.853 +theta=-49800" +to EPSG:32719

but that's not working.

My final goal is to use the custom CRS proj string (or WKT equivalent)
in GDAL and QGIS to be able to transform on the fly the local grid
data to EPSG:32719.

Any help would be appreciated. Thanks in advance.

-- 
Daniel Farfán