Just a thought here... Pre-stage your Alternate or Actual Text in a stand alone FM file. This could be spell checked. Left open, you could copy/paste. Once the PDF is finalized and an associated text file (accessible) is made you would have it available for other possible programmatic approaches.
Spell check the MIF file... You might find this more to your liking.
My "test" - Make the FrameMaker file. Two Anchored Frames, one for each figure. Imported, into each Anchored Frame, an image file. Select Anchored Frame. Right Click, select Object Properties. Select Object Attributes. Enter text. For first figure I used Text Attributes: Alternate: For second figure I used Text Attributes: Actual: Used Save As to save FrameMaker file to *.mif Opened *mif file. Opened Find/Change. Search (Find/Change) on "" if Actual Text is used. or Search (Find/Change) on "" if Alternate Text is used. Open spell checker, spell check the page. Correct as needed. Bring the *mif back to *.fm Check your Alternate/Actual Text entries. You will observe that they are ok. Confirm that Generate Tagged PDF is still selected. Format > Document > PDF Setup > Tags tab Print your PDF. Save As to PDF or to PS then PDF via Distiller. Check the PDF (full accessibility checker). Should be good to go.
I've not had the opportunity to use it but, if MIF usage looks good to you, you may want to look at MIF2Go.
Pre-stage your Alternate or Actual Text in a stand alone FM file. This could be spell checked. Left open, you could copy/paste. Once the PDF is finalized and an associated text file (accessible) is made you would have it available for other possible programmatic approaches.
Be well...