Alternatives to Congo Red?
Yes, congo red bind the cellulose because is a substantive (or direct) dye these dyes don't need a mordant so it binds suddenly to the cellulose.
 There are others dyes for example safflower, cochineal with this property. In biology Congo Red is frequently substitute with Sirus Red F3B and Tioflavone S. At this link you can find some alternatives to Congo Red use for demonstrating amyloid, however I don't know if you can apply these dyes to your case, because I don't find any reference about their use as pH indicator. Although is reported that many substantive dyes are used as pH indicator I can't find any list.

Take in account that acid pigment are not substantive to cellulosic fiber and most of the pH indicator are acids.

I think that maybe you can try to use a mordant to obtain the same result with a pH indicator normally used, however I'm afraid that using a mordant you risk to change the behavior of your pH indicator.