Although not the ideal solution, disabling Request Validation would be the only solution in this case. Since the Form collection is read-only upon postback you cannot alter it to encode the value of the Input field and avoid the validation exception.
On the other hand, you could set RequestValidation to false only for the page in question. Since Items' Texts are predefined, meaning that you know what HTML will be there, there should not be any security risks in disabling validation for the page unless AllowCustomText/EnableLoadOnDemand is set to true.
In this case the user will be able to type some arbitrary text in the input. So, you could use HttpUtility.HtmlEncode at the server to encode the text of the Input. Additionally, if you have any other input fields you could encode their values as well using the same utility.