I use pea plants grown early in the season, after they give out I cut the plants down just at the soil surface and then plant tomato plants after them. Tomatoes for some reason are not adversely affected by the nitrogen in bean and pea plants. I always get bumper crops from tomatoes grown this way.
I have a White flowered hyacinth bean plant but it has become a shrub and grows year round. I would just plant around the roots of the plant, providing you didn't dig them up.

Umh... I was so digging happy I already dug the roots up (It took some effort because they were strong). So, the enrichment value is in the roots?

Sorry, can't address your question, but I wanted to ask you what the roots looked like. Were they tubers?

Lots of great information on Wikipedia about Hyacinth bean. It's apparently a great food plant, "producing purple flowers and striking electric-purple coloured seed pods..... It grows fast, has beautiful, fragrant flowers that attract butterflies and hummingbirds, and it even produces edible leaves, flowers, pods, seeds and roots. Dry seeds are poisonous due to high concentrations of cyanogenic glucosides, and can only be eaten after prolonged boiling.[3]"

Sounds like a fabulous food plant, and I now think I'll grow it next year.

Here is a link that might be useful: Hyacinth Bean: great info on using for food