I am not sure how well Windows Embedded Compact 7 it would perform on the N900; however armv7l is a supported architecture, and the Platform Builder do include sample board configurations for e.g., the TI OMAP 3530. To get it to boot might be quite easy (possible), but in order to fully utilize the device specific hardware device drivers will need to be custom written.

Edit: You can create quite small custom images; Microsoft states that you can create a minimal footprint image taking only 500k space. However, this will increase as additional modules are added to the image installation. Silverlight and Flash 10.1 are supported, performance depends of course on the device specifications.