This is a complex question, but I believe that what you propose is not allowed.
You are suggesting adding hooks into the library to make it easier for you to sub-class the library and thus, at the very least. to bypass the spirit of the LGPL.
The problem is, if you sub-class a class which is subject to the LGPL license in your own code, then your work becomes a work based on the library, rather than a work which uses the library which means that your code is a derivative work that is covered under section 2 (LGPL v2.1) rather than one covered under section 6 (LGPL v2.1). I.e. it becomes subject to the LGPL!
I think that Stephen Colebourne provides a good summary over on javalobby.
I'm not a big fan of knee-jerk talk to your lawyer suggestions, but in this case I think it would be well worth doing so if you plan to proceed with this, otherwise you might be getting a nasty letter from the Free Software Foundation legal team.
Alternatively, you could ask the FSF directly. From their contact page:
For questions about free software licensing and copyright
Please check our licensing FAQ, the license list, general copyleft information, and related pages. If questions remains, email <email@example.com>.
Incidentally, in the related question Reflection and the LGPL, gbjbaanb answers with the LGPL 3.0 perspective.