You can add custom syntax definitions to Textastic by using TextMate bundles. See http://www.textasticapp.com/v6/manual/lessons/How_can_I_add_my_own_syntax_definitions__themes_and_templates.html
I did a quick google search for "SystemVerilog tmbundle", but it looks like currently such a TextMate bundle does not exist, sorry.
Customer support service by UserEcho
You can add custom syntax definitions to Textastic by using TextMate bundles. See http://www.textasticapp.com/v6/manual/lessons/How_can_I_add_my_own_syntax_definitions__themes_and_templates.html
I did a quick google search for "SystemVerilog tmbundle", but it looks like currently such a TextMate bundle does not exist, sorry.