Custom Dynamic DNS refresh rate

[tve]

Minor feature request:

My DDNS provider www.dy.fi requires maximum 7 days address refreshment. I have to manually edit in /etc/inc/dyndns.class file line:
$this->_dnsMaxCacheAgeDays = 25; to
$this->_dnsMaxCacheAgeDays = 5;
after every upgrade.

I wonder if this parameter could be added to custom DynDNS configuration page?

Everything works fine, beside this configuration lost.