Create a text file containing the release number in UDeliver

Yes, this takes some fiddling around I'm afraid.
"Mark up text file" is not suitable, this is meant for the very specific purpose of creating files with target variable placeholders. These are files that execute operations that will run with different var values on different target machines.

There are two avenues leading to a solution here.
1.UDeliver has a feature called "packing slip" (Du. 'pakbon') especially meant for releasing, with your deliverables, a descriptive XML file.
One of the chief pieces of information in this file is the current release version number/name.
Please search Community for "packing slip".
2.If you want more freedom to customise, I would suggest you add a "Run SQL Command" task step that calls one of the USoft utilities for producing a flatfile on the file system.
For example, you can execute: invoke xml.export with select select folder_label, 'file-path' WriteOutputTo from t_dl_release_frame where dl_current = 'Y'. Make sure you have UDELIVER declared as Application in UDeliver, and set Application for the task step to: UDELIVER.
The folder_label of the t_dl_release_frame table holds the current release number/name.
If necessary, you can append XSL to convert the output to the desired flat-text.
Another alternative is to call "select uscsxsl.blend" and pass the pc:WriteFile Blend directive. This alternative takes quite a bit of getting used to... but is very powerful.