Basically, any ARM compatible distro is worth a try, it does not need to be small, just configurable.
I would recommend you go with Arch which installs a minimal system but makes it easy for you to expand it by downloading and installing packages, but the choice is yours.
You can find help on installing Debian on your NAS here and a user here reports successful installation of Arch.
If you are limited to ~100MB, you could try one of the following choices depending on your architecture (you have not specified which arm):
IPFire (the description blurb is from distrowatch.com):
IPFire is a Linux distribution that focusses on easy setup, good handling and high level of security. It is operated via an intuitive web-based interface which offers many configuration options for beginning and experienced system administrators. IPFire is maintained by developers who are concerned about security and who update the product regularly to keep it secure. IPFire ships with a custom package manager called Pakfire and the system can be expanded with various add-ons.
102.5MB ISO image
Architecture: armv5tel, i586
TinyCore Linux (the description blurb is from distrowatch.com):
Tiny Core Linux is a 12 MB graphical Linux desktop. It is based on a recent Linux kernel, BusyBox, Tiny X, Fltk, and Flwm. The core runs entirely in memory and boots very quickly. The user has complete control over which applications and/or additional hardware to have supported, be it for a desktop, a nettop, an appliance or server; selectable from the project's online repository.
Architecture: armhf, i486, x86_64
Linux from Scratch: A very good clear guide and set of instructions on how to build your own Linux from, well, scratch.
Install on a USB stick and boot the NAS from there as described here. I have never tried this so I can't testify as to any possible problems, or whether it will be slow etc. In any case, it is very easy to do so you might as well give it a go and check it out.