Simple Q&A D&D 3.5 (by RAW) XXIII

[Curmudgeon]

A 520b

You are prohibited by RAW from attacking with two weapons while grappling. Armor spikes are one weapon. Thus any round you use armor spikes while grappling you may not use any other weapon. (There is no "reasonable" reading here; just what the rules actually say.) You may use unarmed strikes.

unarmed strike

A successful blow, typically dealing nonlethal damage, from a character attacking without weapons.

A 520a

Armor Spikes

You can have spikes added to your armor, which allow you to deal extra piercing damage (see Table: Weapons) on a successful grapple attack. The spikes count as a martial weapon. If you are not proficient with them, you take a -4 penalty on grapple checks when you try to use them.
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An enhancement bonus to a suit of armor does not improve the spikes’ effectiveness, but the spikes can be made into magic weapons in their own right.

You do not automatically add piercing damage unless you succeed on a grapple attack, and the attack modifier for armor spikes is different from that for your unarmed strike. (The "normal" damage available to a grappler is nonlethal bludgeoning damage. The lethal piercing damage from armor spikes is, from that standpoint, "extra".) The only grapple option available which matches "grapple attack" with a weapon is Attack Your Opponent. Normally this does not allow an attack with a non-light martial weapon, but the description of armor spikes creates a specific exception for their use. Armor spikes, despite being substantially larger than normal light weapons, count as a light weapon.