I don't know if they still do, but Wiggly Wigglers used to sell a 'wildflower meadow' seed sack. Perhaps 'accidentally' dropping/bursting such a bag across the border would start things happening?

Or maybe tethering your goat in the area?

I like both of Lyle and Bronchitikat's ideas.

We have thrown multiple tens of packets of wildflower and bee-friendly plant seeds on the strip over the years (usually the free packets from shows or magazines, or seed heads from our own plants).

Sadly they always come to nothing as the thick grass and perennial weeds already established out-compete the seedlings.

What I didn't know until a friend made a wildflower meadow is that one has to first kill-off all the plant material present (plough or weedkiller).

I think the only option is to threaten to keep our bees locked in so they won't be able to pollinate his OSR and borage at the critical point next year, unless he keeps his field margins neat.

Young Stevo hand-weeds my fields when the ragwort is in flower so that it doesn't seed. One of the fields is cut for hay, so I have to be careful. The quantity of the beastly stuff on the verges of the main road is prodigious, but it sometimes is cut before it seeds.