Good news: I've measured gstreamer memory consumption on n810:
* console app, playing local mp3 file (via dsp): ~2.8mb
* gtk app, playibg local mp3, with working seekbar (GtkScale): ~6.5mb

Since ps command on n810 is very limited, I've measured by VmRSS in /proc/<pid>/status


Btw, I was trying to replace maemo onscreen keyboard with my own back in 2009. I was owned N800 then, so onscreen keyboard was the most important part of the UI for me. But I was stuck trying to embed by keyboard in Hildon Input Method framework. So, the only workin code I had is keys layouting with hboxes and vboxes just like you described