[with atmegascheme more or less in place...]
1] If we expand to 512K say using the 628512 (Hitachi by way of Reichelt) then we need 19 bits for each memory address and thus increases size of list pointers in interpreter. [16 banks 32k (16 bits)]
2] Question of availability of either CC2420 and NRF905 radio chips. Also if we use SPI bus on ATmega128 for communication with either of these.
3] Redesign of main memory board for 512K addressing (latches), fixed programmer header and SPI throughput.
4] Brief look over solar charging possibilities and designs - 2.4v from batteries (2xAAA. say 700 mAh), Maxim MAX1758 and associated voltage reg. chips. See references.
Quick references:
Memory access: http://www.mikrocontroller.net/topic/15889
TinyOS CC2420 codebase
CC2420 board: http://www.rfsolutions.co.uk/acatalog/:_Zigbee_Module.html
NRF905 and SPI: http://www.avrfreaks.net/index.php?name=PNphpBB2&file=viewtopic&t=47356
50mA flexible solar panels: http://store.sundancesolar.com/3vol50maflex.html
[or they have 100mA 3.6v]
Harnessing Solar Power with Smart Power-Conversion Techniques: