Is there a way to dynamically update a list of allowed origins in Web API CORS

I'm trying to enable CORS for certain domains within my .Net Web API application, and am able to do this on Application Start via this code..

public static void Register(HttpConfiguration config)
{
    //below comma separated string is built from database
    var domains = "http://www.myfirstdomain.com,http://www.myseconddomain.co.uk .... about 130 domains more..";
    config.EnableCors(new EnableCorsAttribute(domains, "*", "*"));

However if new domains are added while the application is live, these will not be allowed to post cross domain until the app pool is recycled and this list is built up again.

Is there any way I can update this list during the lifetime of my application? I know I can periodically recycle the app pool but this will cause delays in some requests that I could ideally do without.

I know that I can enable this on the controller method, ie..

[EnableCors("http://domain1.com,http://domain2.com", "*", "*")]
public HttpResponseMessage PostAsync([FromBody] MyRequest myRequest)
{

However again the comma separated parameter has to be declared as a constant, and therefore cannot be dynamic.

Am I missing something blatantly obvious or can anyone think of a decent way of doing this?

EDIT

Here is my attempt at writing my own custom EnableCors attribute..

[AttributeUsage(AttributeTargets.Method | AttributeTargets.Class, AllowMultiple = false)]
public class EnableCorsByDomainAttribute : Attribute, ICorsPolicyProvider
{
    private readonly CorsPolicy _policy;

    public EnableCorsByDomainAttribute()
    {
        _policy = new CorsPolicy
        {
            AllowAnyMethod = true,
            AllowAnyHeader = true
        };

        var originsString = "http://www.test1.com,http://www.test2.com";
        if (!String.IsNullOrEmpty(originsString))
        {
            foreach (var origin in originsString.Split(','))
            {
                _policy.Origins.Add(origin);
            }
        }
    }

    public Task<CorsPolicy> GetCorsPolicyAsync(HttpRequestMessage request, CancellationToken cancellationToken)
    {
        return Task.FromResult(_policy);
    }
}

I've then decorated the controller method with

[EnableCorsByDomain]