Actually textobject thumbs work like video thumbs, which means default image if you didn't upload an special image with the same name (minus suffix) as the textobject. The default ones arae not cropped but the thumbs images should behave like any other image/thumb.
Please try the 1.4.3 nightly if you have the change. The rss feeds have been internally reworked a great deal and a few things have been fixed/improved. Those will not likely get into the 1.4.2 stream anymore. (Note the rss links change a bit, see zp-core/class-rss.php top comments in the 1.4.3 nightly about that).