We looked at a similar job a while ago and the seagrass had been cleaned before using sponges but the customer wasn't happy with the results, he also said a lot of the sponges had not come out of the weave and it had taken them weeks go get them all cleared out.
I walked away from they job, but it's just something to watch out for.
I believe seagrass can also be bonnet cleaned, anyone used this method?