I am not Stefan, but maybe I can add to this issue.

Personally I think it is not a good way to dump all the information into the text field and try to let the server parse it.
This could be archived more simply by using e.g. the python elog scripts or using the elog command tool to directly submit well structured elog entires.

As you have mentioned you could use the "execute new|edit|delete" commands to do the things you want to do.
Let the called script fetch the elog entry and alter it in the way you like and resubmit the entry.
(You have to make sure to don't generate a endless loop and you have to proably have to use a forwarding script as the elog "execute" command waits for finished execution).
This way you don't touch the 1500 characters limit of passing $text to the execute command.

Also the point "filling an attribute X with free text" needs some kind of (extended) parsing, which isn't possible with the subst command.
In our elog we such an attribute X, which is in some conditions filled with a set of other attributes and on other conditions just a normal input field, so the user could type custom text in.

"fill an attribute automatically with the character length of the text" could be done with the above mentioned execute command or it could be done on the clientside (e.g. browser using javascript or if automatically generated by a script, then directly by the script)

I hope it helps to build our application.
Best wishes,
Sebastian