Only if the three CV solution proves too difficult, an alternative method, which has been used successfully on a biennial long-distance race, that has several consecutive starts in the Solent, has been to position a RIB around the mid-point of the line. Having pinged both ends of the line the RIB patrols the middle sector of the start line using GPS as a guide. Just before each start said RIB will then turn onto the course and manœuvre to keep clear of competitors as they sail past.

Another method is to have a mid-line buoy.  It is set after the signal and pin vessels are anchored, about 5 to 10m downwind of the starting line.

The SIs should be written that the buoy is "nothing" - it is not a mark of the course, nor is it a part of the starting line.  It's just "there." No redress for snagging the anchor line with underwater appendages.

This worked very well at Sunfish Worlds, where there were 100 nearly identical boats on a line 0.3 nm long.

You can also use this diagram in the change to RRS 30.1. Good luck!

In the Star class we have used a system with a RC vessel in the middle and boats in each end of the line

The following wording has been used:

Quote
11.4 The starting marks will be the RC Signal boat at the center, a boat at the starboard pin end
and a boat at the port pin end.

12.3 The starting line will be between staffs displaying orange flags on the starting marks, which
will consist of three (3) RC boats: port end line boat, mid-line Signal boat and starboard end line
boat. It will consist of two segments – one between the port end line boat and the mid-line Signal
boat and the other between the starboard end line boat and the mid-line Signal boat.
12.4

12.5 When RRS 30.1 (I flag rule) is in effect, boats subject to this rule shall thereafter either:
(b) sail from the course side across an extension of the starting line beyond either the portend line boat or starboard-end line boat to the pre-start side of the line
or
(b) round the mid-line Signal boat before starting, leaving it to port. This changes RRS 30.1.
Unquote

Hope this is usefull.

A regular feature of the CIE Optimist Event (International Summer Cup) in France. 400+ boats for a coastal race (yes, you heard correctly!)  SIs in French here https://drive.google.com/drive/folders/1DGqkqE3sk9DocwkCFUjcUmOc4f_A4kFR 

Thanks All,
Have gone with the Danish version (thanks Hans) but changed boat to vessel. Will not be using I flag so no need to add that complication.

One advantage of defining the line as 2 separate segments is that the line on each side of the mid-line vessel to not have to be the same. You can adjust the lines on each side so that they are suitable for the existing conditions. On a long line it may well be that wind or tide will not be uniform over the full length of the line.