I will give this some further thought but the standard answer would in tiddlywiki be to divide into it here tiddlers. Have you used the excise tool? You mention not splitting but I am not sure if you considered excise.
However since the two things you seem to need is to indicate a section of text and search for it later I can imagin a lot of ways to do this.
Rather than define your requirement by reference to OneNote perhaps you could describe what you want to see and do, be imaginative and dream of the ideal and let us know. I / we are all about making dreams come true in tiddlywiki.
Regards
Tony
If you want to do it yourself!
Many ways are available
You can use hidden comments the content which can be found using search.
You can use html sections, div or span to wrap a section
You can use css classes on such sections
You can use splitregexp[\n] to extract lines from text the prefix[string] to find lines beginning with something.
If you are happy to use a particular existing wiki text standard e.g. headings you can build a tool to list it when found in tiddlers.
You can make text as a parameter to a macro.
You can wrap text or a transclude inside a button for click to action.
Etc etc...
Regards
Tony