Well it seriously depends on what type of grunge you are going to be playing.
Alice In Chains style would be just like metal, like Pantera but a bit slower and bluesier.
Soundgarden style would just be like any rock solo, Pearl Jam too. A scale. If there's anything that seperates grunge solos from just general rock solos by Kiss or Led Zeppelin, it would have to be irreverence. That punk-rock attitude. That means be sloppy if you want to be, never be concerned with speed at all, and do a lot of feedback and "noise solos" where you just hit a bunch of random notes and make feedback noise. A good sounding bit of feedback can be every bit as good as a guitar solo, especially in a grunge song.
The key to Nirvana soloing would be to be as simple as possible. Still tasteful and cool, but as simple as possible. It's like old Johnny Cash stuff - just a handful of notes in a repeating sequence. I mean, Come As You Are's only has 4 notes and that is the way a lot of Nirvana solos are. I can't exactly tell you exactly what to do, but just look at Nirvana's solos and it will easily come to you. It's not essential that you play with any technical assurance. Just some notes in a cool pattern that fits the song.
And anyone who thinks grunge doesn't have solos is stupid. Virtually all rock ever has solos. Well over half of Nirvana's songs have solos. I can't even think off of the top of my head ANY Nirvana songs that don't have solos.
One other thing you can do is just play the vocal melody with notes. That is exactly what Smells Like Teen Spirit's guitar solo is. It shouldn't be hard to figure out what notes to play for this, just play whatever sounds like the vocals. If it's not exact it will still have the EXACTLY right sound that a grunge solo should have. Unless you're singing a Slayer song because then the solo would sound too fast...