Reporting back . . . I stopped by my favorite Indian grocer late yesterday (not convenient generally, but it's on the way back from other occasional errands like getting my car serviced; otherwise, I'd have gone sooner).
Conversation goes something like this (Picture AntoniaJames scooping toor dal in the bulk section):

Are you finding everything you need?
Yes, thank you, but where are the fenugreek seeds? [The ones I have at home are of an unknown age.]
Oh, right here. (Guy in store starts scooping a little bag for me.]
Do you have fresh fenugreek?
Sorry, it's not the right season. It's too rainy now.
When do you think you'll get some in?
When it stops raining completely for at least five days.
Oh, that's great. What would you do if you didn't have fresh fenugreek and you needed it for a recipe?
Frozen!
Wow, do you carry that?
Sure. Let me get some for you. [Looks deep in refrigerator case, says he'll need to get some from the back, and 30 seconds later, hands me a bag of it!!]

So, now all I need is some idea of how the frozen stuff compares to the fresh, in terms of amounts.
I'll pose that one to the cook who posted the recipe here.
Stay tuned for more on that, and for a report on the recipe!!
Thanks, everyone. ;o)

Pretty distinct flavour, so rather than substitute, I'd wait until you can find some fresh methi. Try the indian grocers. There are so many now in the US, it shouldn't be that difficult!

Fenugreek is really easy to sprout, maybe try using chopped sprouts instead? It would make the recipe take about a week longer though.

Maybe add some dried fenugreek leaves in addition to the watercress mentioned.

I'm not an expert on this but you can have my two cents. Fresh fenugreek is something distinct and not the same flavor as the seeds. This recipe is Methi Dal, so the fenugreek is the prominent seasoning. Is is likely available in season in a good Indian market. But any dal is delicious, usually. I read a comment on chowhound that watercress was a reasonable substitute.