Well interesting approach I wonder how you figured this out.
Here's a simple approach. Correlate viewstate and probably other things prior to the page you expect to use it on.
web_reg_save_param data is actually stored "correlated" after the request has been made.
For example see steps 5&6 below. Step 8 is where the viewstate would be needed.
1. web_set_max_html_param_len .... set to the longest string you expect
2. web_reg_save_param("VST:,LB=VIEWSTATE\" value=\"", "RB=\"" ..... setup to capture view state
3. web_url("default.aspx", ... open the home page..
4. lr_message( ... display the value of view state here .. did you get it? )
5. web_reg_save_param("VST:,LB=VIEWSTATE\" value=\"", "RB=\"" ..... setup to capture view state
6. web_link("Adept",
7. lr_message( ... display the value of view state here .. did you get it? )
8. web_submit_data("search.aspx", *use your VST in the viewstate param.