Here are some leads, but I was unable to solve your problem.
a. Using caption on a frame containing the formula
Unfortunately, frame captions can only be inserted above or under the contents.
Second, you can’t fully customise the ‘category’ auto-inserted text, i.e. you can have this caption text before the number (as in Formula x), not around it ( first part (Eq. before, closing parentheses after the automatic number).
b. Defining a new paragraph style with tabulation
I can now have text at left margin, a center-tab at centre for the formula and a right-flush-tab at right margin for the number but I don’t know how to have this latter attached to any list to get auto-numbering (and eventually generate an index/TOC)
c. Work-around based on b.
The nasty trick is to use the line numbering feature but you’ll lose the ability to number your lines because the feature is used for something else.
- Modify “Standard” style (Outline & Numbering tab) to uncheck “Include this paragraph in line numbering”. This will propagate to all styles.
- In your custom formula style (see b.), check “Include … line numbering” box.
- Activate line numbering in menu Tools->Line Numbering. Set Position to Right and Interval to 1 so that every line gets a number.
You can now use your custom formula style as follows: an optional short title or id (at left), tab, your formula, tab, a fixed caption “Eq.” (without parentheses since you wouldn’t get one after the line number).
The sequential line number (= your equation number) is automatically inserted by LO inside the right margin.
Your equation is limited to a single line, otherwise you have multiple numbers.
You can’t capture these numbers to build an index/TOC.
You can’t reference these numbers from the text as you would for outline numbers (so that they’re automatically adjusted when you change the order of the formulas or when you insert a new one).