After sleuthing some maps in Gaia GPS, a means of access was determined for Green Mountain. Prior reports cast some doubt but the route provided is all on public land, Washington DNR to be specific. On my trip, the outer gate was locked but the land is not signed as prohibited for access, so i hoofed it in. If you park at the gate, don't block it and bring your Discover Pass.
The gate is at (45.95303, -122.50253). From there, proceed uphill, following the track provided in SOTA Maps and SOTL.AS. Most of the hike is on forest roads with a small section of single track in the middle.
The summit is forested but the road runs right through the AZ. Ample space is available for any kind of antenna. Contacts into the PDX Metro via 2m are possible. Bring a half wave whip ideally, or J-Pole. Cell coverage via AT&T was adequate.
GPX file: https://drive.google.com/file/d/1capi3mhOvZJFF3q6sb_q9zB5wweZHbpD