Brother Joukahainen,
Are you working with White Ash by chance? It looks like WA and WA is known for tunnels like that. I have worked with a few that had these kinds of tunnels.
I'm not any kind of expert bowyer, but here is my advice.
#1. Do not leave it alone and try to just tiller around it. Too bad I know, because that would be the least stressful option. But, sorry, you will have to do something you cannot just leave it.
#2. Scraping it out, down to another ring or two would be the best option to deal with it, but if that would not permit you to get your 70-80lbs, then we will kick that one out, too.
#3. OK. So you have to have your 70-80 and you can work down any more growth rings. You have to fill it. It will not be a big deal and you can even sand it down when you're done and stain the bow and no one will ever even know it was patched. You have some choices here too, as to what you fill it with. Fortunately, this worm tunnel is more cosmetic than anything, and even if you left alone wouldl not take much away from your bow, or even affect the tiller all that much. The main reason you must fill it is to avoid splinters and an obvious weaker spot.
Have you ever worked with liquid wood, or wood putty (it may be called)? It comes in a little can, and ends up drying very hard. You can mix water with it and fill it layer by layer, packing it in as much as possible and then sand it over. Another, and possible cheaper filler, is like you said-very fine sawdust with glue. A way I have found and actually prefer, is NATURAL sinew. From a deer or whatever you can get it from. And you don't really need all that much. If you have not worked with sinew before, let me know and I can explain about it as well. I personally like sinew because it has strength value and get's very hard and will feel like actual wood after a few days. If you do fill it with sinew, you will still have to sand over it to smooth it out once it is all dry, and maybe put a small layer of sawdust and glue over it and then sand that so that you don't have to actually sand the sinew itself. Either way, your tunnel CAN be patched, my friend. Don't scrap your wood if it's good wood.
Let me know what you decide. Hope I have been able to help.
--Ol Matt