An assembly supports a control line of a downhole tool, such as a downhole deployment valve. The tool deploys on casing and has a control line extending from the wellhead to the tool. To support the line separately from any casing hanger, a split bowl disposes around the casing and lands on a shoulder in the head. A port in the bowl has one opening that aligns with a side port in the head. Another opening of the bowl's port connects to the control line that extends to the downhole tool. A section of the control line from the split bowl can be flexible to help prevent kinking or breaking of the line during installation procedures. A hanger disposes on another shoulder in the head uphole from the bowl. The hanger supports the casing in the head separate from the bowl's support of the control line.