What should be done if the Zakat amount contains fractions, and small-denomination currency cannot be paid, is it permissible to round up the amount with the intention of charity, or does that invalidate the Zakat?

There is no objection to giving an extra amount with the obligatory portion of Zakat; it is considered a supererogatory charity. The extra amount might become obligatory if Zakat cannot be given except by paying it, due to its inclusion under the principle: "That without which an obligation cannot be fulfilled becomes obligatory in itself, as an ancillary obligation."