How to flip partial text without converting or rasterizing?

Perhaps you can accomplish your task using cloned smart objects? This technique requires that your text is always same length but you can find and some other solution along the way. Here is idea:

• Type text on type layer then convert it to Smart Object.
• Duplicate Smart Object using Ctrl/Cmd + J. That will create cloned smart object. When you update content on any of cloned objects Ps will automatically update parent/child SO layer. Transformations does not count, any transformation and masking is individual.
• Mask parent SO layer to hide last letter.
• Transform child or duplicated SO layer so letters appear as you want them: flipped.
• Mask all text beside last letter on duplicated SO layer.
• Double click parent SO layer and change text.
• If you expect to have different length of text then try with allignment options or try to align right side of text to guide or document edge. It is not perfect but I can not think better solution at the moment. 

If I rasterize or convert it will do the entire line or need a new layer,

Not necessarily. Rasterize the type, Select the last letter, then Edit > Transform > Flip Horizontal. Use the Move tool to postion the letter with greater precision. 

I suggest you place that character on it's own text layer and move it accordingly.  I have no other solution, sorry.