In the event that a U.S. state guard is ordered to federalize could they refuse that order? Is there any justifiable reason they could give for not following that order?
This is addressed by state law. I pulled a couple of arbitrary examples below. In both cases, state law puts the state guard under the control of the Governor, subject to compliance with federal law.
Connecticut - Possibly
The Connecticut State Guard doesn't have a permanent presence, it is activated at the request of the Governor. If that happens, citizens in the unorganied militia can be mobilized (Sec. 27-9).
Although the Governor is the Commander-in-Chief of the State Guard (27-14) and no provision exists for transferring that command, the State Guard is subject to the federal draft. Additionally, any commands or regulations for the State Guard must be in compliance with federal law, so if federal law allowed them to be "federalized" than it would be allowable.
Oregon - Possibly
Oregon state law says:
Whenever laws of the United States authorize the organization of such state forces under federal recognition, the Governor shall promulgate such regulations as are necessary to comply with such federal laws and obtain federal recognition for the force authorized by this section.
This seems to indicate that the state of Oregon anticipates that the Defense Force can be "federalized". However, the Oregon State Defense Force is that all members are required to be ineligible for military service under the selective service program. So if this eligibility is required for service, they wouldn't qualify.
Both states defer the issue to federal law. Wikipedia contains a neat (and well referenced) summary of the relevant federal law. Hihlights: there are several cases where federal law allows the President to utilize state militias (a term which includes the state guards, at least in the states I looked at). However, there is an extant constitutional issue regarding whether such units may be called into the federal armed services. This has not yet been decided by the Supreme Court.
The National Guard is a formal part of the (federal) Army and Air Force components of the US military. They can be activated (federalized) by orders of the POTUS. As the POTUS is the commander in chief, all guard units answer to the POTUS.
As such, the only reasons they'd have to not follow an order would be the same reasons as any soldier would have for not following orders: that the order is unlawful. There are examples of presidents issuing illegal orders: https://sites.duke.edu/lawfire/2016/03/10/have-presidents-ever-given-the-military-illegal-orders-yes-the-surprising-listand-more-about-the-law-of-military-orders/
As for State Guards, they are not part of the federal military and, as such:
State defense forces are distinct from their state's National Guard in that they cannot become federal entities.
As such, there'd be no order to federalize--so no order to refuse.
According to 32 U.S. Code § 109 - Maintenance of other troops:
(c) In addition to its National Guard, if any, a State, the Commonwealth of Puerto Rico, the District of Columbia, Guam, or the Virgin Islands may, as provided by its laws, organize and maintain defense forces. A defense force established under this section may be used within the jurisdiction concerned, as its chief executive (or commanding general in the case of the District of Columbia) considers necessary, but it may not be called, ordered, or drafted into the armed forces.
The same law, however, does permit drafting of members of a State Guard:
(d) A member of a defense force established under subsection (c) is not, because of that membership, exempt from service in the armed forces, nor is he entitled to pay, allowances, subsistence, transportation, or medical care or treatment, from funds of the United States.
Conscription isn't explicitly mentioned in the US Constitution, but the Selective Draft Law Cases ruled it permissible. If this were attempted in order to undermine a specific State Guard, there might be objections under basic principles of federalism.