So I take some of that back. I've just killed off syllabics altogether, so you can ignore the bit about them being their own letter à la icelandic and all that.
C[+sonorant,+syllabic] → C[-syllabic]a
Syllabic sonorant consonants become non-syllabic and are followed by -a.
swistr → swistra (sister)
rign → rigna (rain)
It also has some interesting implications for verbs:
dūn, stēn, gīn - no syllabics here. But:
wisan > wisn > wisna
waírþan > werðn > werðna
Here's also a nifty twist for Class 1 weak verbs:
wiljan > wilin