The jackets that are REALLY lightweight and seat-bag-stuffable are usually more like windbreakers. They might work well for light rain though. Try looking at the "Whisper Jacket" on TE. I find Gor-tex jackets to be too sweaty and non-breathable, though they certainly do keep the rain out. If you have a small seat bag you might have to stick to water-resistant windbreakers that roll up real small.
Mimi has a great rain jacket....Mimi?