The choice of sequestering agents that can be used in an ascorbate developer is very limited. EDTA, for example, will actually promote the oxidation of the ascorbate ion. This has been menthioned many times and by Ryuji himself.
Oxidation of ascorbate should be a non issue if you mix fresh from concentrate and use right away. I am aware of Ryuji's statements but unlike Mark's recipe Ryuji's published recipes are not concentrates in PG. The sequestrant I suggested should at least take care of Ca2+ and Mg2+, therefore I suggested Na2-EDTA and Calgon.
Trying to be the best of whatever I am, even if what I am is no good.