Hello
This is possible. Set up a hotspot on your Mikrotik device and redirect login requests to your website by changing the login.html file like this:
<html>
<head>
<title>Please wait</title>
<body onload='connexion.submit()'>
<h2 align=center><i>Please wait</i></h2>
<form name='connect' action='http://www.your_portal.com' method='post'>
<input type='hidden' name='sip' value='$(server-address)'>
</form>
</body></html>
Of course, http://www.your_portal.com must be in the hotspot walled garden.
When the registration/login is complete, http://www.your_portal.com must redirect the client to the local hotspot page using a page like this
<html>
<head>
<title>Please wait</title>
<html><body onload='connexion.submit()'>
<h2><i>Please wait</i></h2>
<form name='connexion' action='$initial_login_URL' method='post'>
<input type='hidden' name='username' value='$login'>
<input type='hidden' name='password' value='$password'>
<input type='submit' value='OK' name='OK'>
</form>
</body></html>
Where $initial_login_URL is the mikrotik hotspot address, which was passed in the $(server-address) parameter.
Another option is to get a mikrotik device online at your ISP, reachable from the internet, and create EoIP tunnels from your APs to your controller to redirect all client traffic.