I have come across some chain aimed at the model boating market which may be suitable:
Caldercraft (http://www.jotika-ltd.com) produce a "solid link chain 4 links per inch" sold in 1m lengths for £12 (part no. 65432) It appears to be welded, but I have not yet tested its strength.
I enquired whether they supplied chain with slightly bigger links – they said no, but could manufacture any size if the minimum order was 200m+ !!
I hope this info helps other builders.
David Greenaway